Hi tiredben,
The better options I’m aware of in the Nashville area are listed in post #7 here. While it may not be the “best” area … there are certainly some better options there. There have been a few members of the forum that made some very good quality/value purchases of Dutch Craft mattresses for example.
Each manufacturer knows much more about the specifics of all the layers and components of their own mattresses and which of the models they make or options they provide would be most suitable for different body types, sleeping positions and preferences based on the “averages” of their customer base. There is a lot of information about some of the theory behind different layering and designs and how they relate to body type and sleeping position in the mattresses section of the site but these are more generic and meant to help you understand the ideas behind mattress layering and the manufacturers or retailers themselves are always the best source for more specific suggestions for their own mattresses and the “averages” they use for different people.
Some of the better online sources for memory foam mattresses I’m aware of (including Rocky Mountain mattress) are listed in post #12 here. The more local testing you have done on mattresses where you know the details of the layering, the better you will be able to use your testing as a guideline for an online purchase.
Phoenix