Designing and building your own mattress can be a lot of fun but can also be quite frustrating and involves a lot more knowledge of how different materials, layers, and components interact than many people suspect. Posts 15 - 17 here may be worth reading and I would normally recommend working closely with a manufacturer who will be able to give good advice about all the different components (option 2 in the linked post) over using your own knowledge and separate sources to build your mattress (option 3 in the linked post) but for those who want to forge ahead with their own design anyway … this will provide some options.
Ordering layers separately and where to order would depend on the material you wanted. Local mattress manufacturers and local foam shops can often be good quality and value sources if you want to see what you are buying first or talk to someone that has the skill and knowledge to help you make good choices.
NOTE: I would also be cautious about sizing because toppers can be cut to smaller sizes or have size variances that may be smaller than is listed on a website or than mattress components or bare layers that are meant to be used inside a mattress with “standard” mattress dimensions so if more exact sizing is important to you or you are ordering mattress components for inside a mattress instead of a topper then I would always confirm the actual dimensions of the layer, topper, or component you are ordering with the manufacturer or retailer you are ordering from.
There are several good sources for latex toppers and/or full cores. My reference points are a few of the members of this site …
https://tmu.best/asb07 3" Dunlop & Wool toppers
https://www.mattresses.net/diylamaco.html Talalay and Dunlop (incl. slow response Dunlop)
https://www.matt-to-go.com/ Pure Talalay Bliss Talalay latex toppers
https://www.sleepez.com/ Talalay and Dunlop
https://en.memoryfoamcomfort.ca/main.html Organic Dunlop (Canada)
These include a wide range of Talalay and Dunlop layers of several types and thicknesses and are all recommended members of this site which means that I believe they have among the best quality and value in the country. Some other good sources include …
https://www.kttenterprises.com/index.html . Any size, thickness, or ILD for Talalay Global Talalay and well priced per board foot.
https://www.sleeplikeabear.com/ Has any size and thickness and ILD of Talalay latex made by Talalay Global.
https://www.foamorder.com/ Also sells Talalay and Dunlop and their Dunlop can be ordered in 6" cores in a variety of 5 firmness levels. Also, check their clearance items.
Mattressess247 here Blended Talalay and 100% natural Dunlop (including zoned Dunlop cores) with ebay https://stores.ebay.com/mattresses247
https://www.ebay.com/usr/latextoppers Sells latex cores and toppers including organic Dunlop from Latexco that are accurately listed.
https://stores.ebay.com/organicandmore/ Natural and organic Dunlop.
http://www.organicmattressshop.com/latex-toppers.html Convoluted latex topper.
Google searches on ebay, amazon, and other similar sites and sometimes in the big box stores can sometimes bring up more alternatives and sometimes lower prices but again it’s important (to me at least) to know that what you order is what you get and that the vendor has a good reputation and integrity. In some cases, you will find comfort exchanges or seconds on these sites (sometimes identified and sometimes not) and sometimes the products will be either poorly described or mis-described completely. If you are OK with this and you trust the vendor (or they have a great refund policy) then they can be a good source as well.
FOR SHREDDED LATEX TOPPERS:
Shredded latex toppers can “displace” more than solid latex layers and provide pressure relief with less risk to alignment. Some sources for different versions of these I’m aware of include …
https://www.cozypure.com/mattress-pads-toppers/lanoodle-cuddle-top-latex-topper-best-seller see this post for more detailed information and feedback
https://www.flobeds.com/products/linens/mattress-toppers.htm (has a zipper so the shredded latex can be added, removed, or zoned in different areas. See this post for more detailed information and feedback).
Again local foam shops are usually a good source for a wide range of polyfoam in terms of both information, quality, and firmness but online there are also several good options that will accurately describe their foams and their qualities and have good prices. A few of these include (pricing hasn’t been compared) …
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http://www.foamonline.com/ (they also sell Talalay latex but be very careful that you receive the correct ILD)
Phone for prices:
http://www.newenglandfoam.com/ (min order $200 last time I talked with them)
Some of these may also sell other types of foam including memory foam, gel memory foam, or latex and many don’t compress their foam for shipping and will only ship foam layers that are either thin or small enough to fit the UPS size restrictions so phone first to check if they can ship the size and thickness you are looking for.
FOR MEMORY FOAM:
There are so many different types, qualities, and properties of the many types of memory foam that knowing what you are buying depends on the knowledge and description of the merchant you are buying from. It is also often the most mislabeled type of foam and low density or uncertified memory foam is being sold everywhere with descriptions that are misleading or in some cases completely wrong. They are also more widely available because they are a common topper material and only used in comfort layers and never used as part of the support system of a mattress.
Some (of the many) better sources of memory foam layers that carry more than one type of memory foam are in post #12 here and includes …
https://en.memoryfoamcomfort.ca/main.html (A recommended member of this site … Canada)
Some other links to online outlets that are not foam suppliers and may have little knowledge of the quality or specifics of the memory foam they sell but do sell memory foam toppers are in this topic.
FOR GEL FOAMS:
You can read more about the different types of gel foams in post #2 here. The majority of the gel memory foam toppers available on the internet are the type that uses gel beads or particles in the memory foam.
https://www.sleepwarehouse.com/ Gel memory foam
http://www.lakemattress.biz/Toppers.html Gel memory foam
Gel memory foam (3 lb only)
Solid gel on top of either HR polyfoam, memory foam, or latex
http://www.sleepart.com/ Solid gel on top of either memory foam, polyfoam, HR polyfoam or latex
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FOR BUCKLING COLUMN GEL
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FOR WOOL TOPPERS:
Post #3 here has some of the better sources and more information about wool toppers for those who want to add a wool topper as a layer of their mattress. All of these are knowledgeable and informative about their products on the phone.
FOR MICROCOIL TOPPERS:
microcoil with a quilted organic cotton cover
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https://www.ikea.com/us/en/catalog/products/80298172/ microcoil with a 1.25" 1.5 lb polyfoam layer on top of the microcoil.
https://www.therapedic.com/bedding-and-home/mattress-toppers/therapedic-3-5-inch-micro-coil-mattress-topper/ Microcoil/memory foam topper.
https://www.naturepedic.com/our-products/pads-toppers/toppers/3-essentials-pillowtop Microcoil topper.
https://www.berkeleyergo.com/posturflo-pad/ MicroCoil/Latex topper.
https://www.mattresses.net/copocobylean.html This is a high quality zoned pocket coil with firmer rows of coils to provide edge support. They are also a member of this site.
Both of the following innerspring manufacturers have also told me that they would ship an individual spring unit for those who are willing to pay the shipping (which can be costly).
http://www.texaspocketsprings.com/contact.htm Prepay with cheque or possibly Paypal.
https://www.hsmsolutions.com/ (Hickory Springs)
FOR MATTRESS or TOPPER COVERS:
Mattress covers can be one of the most significant parts of the cost of a mattress and also one of the most important. Not only can they have a major influence on how a mattress looks and performs … cheap or thin mattress covers will not properly protect the foam inside and in particular with latex will allow the foam to degrade and break down much more quickly and significantly lower its longevity. Some of the better sources of zip mattress covers include …
https://www.sleepez.com/mattresscomponents.htm (a member of this site)
https://www.mattresses.net/ (a member of this site)
https://sleeponlatex.com/products/organic-cotton-cover-1 (a member of this site)
https://latexmattressfactory.com/ (a member of this site)
https://en.memoryfoamcomfort.ca/main.html (a member of this site … Canada)
sleeplikeabear.com/mattress_cover (good selection)
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https://tmu.best/ekhwl (including an “expandable” organic cotton knit ticking)
https://www.foamorder.com/mattress-cover.html (several covers including quilted latex)
https://mattresses247.com/ or http://stores.ebay.com/mattresses247/ (knit and quilted)
https://www.purerest.com/ (not listed so a phone call and quote is needed)
http://mattresscover.com/ (has access to covers that are not listed on the site so a phone call may be necessary)
http://www.supremequilting.com/mattress-covers-c1 In Canada.
OTHER SUPPLIES or DIY PILLOW SUPPLIES:
https://www.cozypure.com/miscellania/diy-materials-and-fabrics Wool batting, cotton fabrics, cotton batting, lanoodles latex.
https://tmu.best/ekhwl Wool batting, wooly bolas, cotton fabrics, inner and outer pillow cases, kapok, millet hulls, shredded latex.
whitelotushome.com/all-products/wholesale-fibers/ Wool batting, cotton batting, buckwheat hulls, kapok
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https://www.foamorder.com/accessories.html Shredded foam/memory foam, down/feathers, polyester pillow stuffing, dacron wrap, pillow fabrics/ticking.
https://openyoureyesbedding.com/ Buckwheat hulls, millet hulls, pillow cover.
http://www.ecochoices.com/ecobedroom/bedding/pillowprotectors.html cotton/wool cover.
http://www.beanproducts.com/mm5/merchant.mvc?Screen=PROD&Store_Code=001&Product_Code=7ZFFOL Shredded latex.
Most upholstery stores or foam suppliers will carry upholstery glue that can be used to glue foam together but you can find it here and here and here as well.
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Some additional sources for zip pillow covers are in post #8 here.
Of course, these are not a complete listing and google searches specific to the material or component you are looking for will bring up more sources but it should be enough to provide good quality sources for most of the materials or components that are needed.
Bedtimes magazine also has a list of suppliers for different types of mattress components in their supplies guide here for those who are looking for other types of materials or components that may be harder to find at a consumer purchase level but many if not most of these may be wholesale only and may require some research and phone calls to find out if any of them would sell in small amounts to consumers.