Hi Dreamer.
Nice to have you back to our Mattress Forum! Long time no see …. since 2013 
You are certainly throwing many variables, unknowns, and a few health concerns in the “pot” and I would strongly advise to do a hard reset and rethink the way you approach this, both because you seem to be in a rush to be done with it which is indicating that the “spirit of adventure” is missing and because you have too many preference questions unanswered. There will be a lot of trial and error in building your own DIY and because it is not clear to you what you really want, the first thing I would do in your case is to slow down, do a bit for research and answer a few of the questions you have that only YOU can answer as only you can feel what you fell on a mattress and no “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here
A DIY will not necessarily save you money as it can be quite frustrating at times and involves a lot more knowledge of how different materials, layers, and components interact than many people suspect. You cannot be in a rush with a DIY as there are too many variables that you are considering at the same time and without having a firm baseline of understanding of what your real needs and preferences are (as your many question marks indicate) this can only make things confusing, blur out any clarity you may have and be less likely to be a successful project.
Caution about building your own DIY… lots of trial and error…
If you are attracted to the idea of designing and building your own DIY mattress out of separate components and a separate cover then the first place I would start is by reading option 3 in post #15 here and the posts it links to (and option #1 and #2 as well) so that you have more realistic expectations and that you are comfortable with the learning curve, uncertainty, trial and error, or in some cases the higher costs that may be involved in the DIY process. While it can certainly be a rewarding project … the best approach to a DIY mattress is a “spirit of adventure” where what you learn and the satisfaction that comes from the process itself is more important than any cost savings you may realize (which may or may not happen).
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress inpost #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” that may be useful as well.
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 post #15 here) over using your own knowledge and separate sources to build your mattress (option 3 in the linked above) but if you want to forge ahead with your 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.
Regarding component suppliers, while not inclusive, there are some options listed here , with a few Canadian sources here . Unfortunately, I don’t keep a record of the individual component suppliers in the hundreds of lists throughout the forum or have available online, as it would be a bigger job than anyone could keep up with in a constantly changing market.
Once you have a chance to clarify some of the personal questions that only you can answer and perhaps “sketch out” a simpler design please let me or any of our Expert Members know and if you have more specific questions we’ll be happy to assist you.
Phoenix