Hi howardsbabygirl,
In general there are two main approaches to buying a mattress.
The first involves “becoming an expert” and the second involved "finding the experts. You can read more about this in post #4 here.
There are also 3 main food sources of mattresses which are local, online, or becoming your own mattress designer and buying the components to make your own. You can see more about this in post #15 here.
The direction you are thinking of is option #3 which would involve more “becoming the expert” and would generally need well above average knowledge and experience in the different types of foams and components and enough experience with different layering combinations to have the confidence that what you are designing and building would work in “real life” as well as it does in theory.
This is by far the riskiest option (and can be the most costly if you make mistakes) and if you have exhausted your local possibilities and can’t find the type of mattress and the quality/value that you want and are looking online … then who you work with can still be even more important than what you buy.
The first post I mentioned has some links to sources but if you are looking for some better online memory foam options (including components mattresses where you can choose your own layers or re-arrange them if necessary) then post #12 here will help. This includes some of the members here which specialize in memory foam mattresses.
There is also a complete list of the members of this site who specialize in online (or on the phone) sales and service and have exceptional quality and value listed in post #21 here (including the members that were in the memory foam list).
So my “best” suggestion would be to “find the experts” rather than trying to design and build your own mattress on your own piece by piece and if you aren’t able to find a suitable mattress locally that has the kind of quality and value you want, then to work with an online manufacturer you can trust (along with what you have learned from your local testing) who can give you the best of both worlds which is the ability to choose a mattress that has the design that you (and they) believe is best and using their experience, knowledge, and guidance to help you make better and more informed choices.
Phoenix