Hi NeedHelpPlease.
Welcome to our Mattress Forum. 
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 an eCommerce retailer/manufacturer that has the skill and knowledge to help you make good choices.
Your proposed 12" construction could work well but you would need to consider it against the “specs” about your body type and sleeping position which are an important part of how each person interacts with a mattress ( there are many other variables as well). There are many different types of layering combinations that can perform equally well for people of a certain height and weight and sleeping style so the most “accurate” way to find a good “fit” is to deepen your research and read the general weight and height guidelines here and some general guidelines about sleeping positions here and some information about how different types of layering can change and affect these guidelines in this section of the site but these are general guidelines and starting points only and not be used to design or recommend a specific mattress based on what I call “theory at a distance”.
Hope this helps a bit for you to get a good sense of things about DIY.
Phoenix