Hi chip,
As far as the NY area list goes you may be right except perhaps on a store by store basis where a particular store may carry a mattress line or model I’m not aware of (the forum lists are certainly not all inclusive of course because I don’t know every product sold by every retailer in the areas I research and in many if not most cases I have found store websites are not up to date which is why I talk to them to find out what they are currently carrying as much as my time permits).
There are a few memory foam wholesale manufacturers that use Chinese memory foam that are CertiPur certified and are usually transparent about the materials in their mattresses (although this may depend on the retailers willingness to call their sales rep or the factory) but I would evaluate each mattress you are considering to make sure it meets the guidelines I listed earlier in terms of foam density because some of them may also use lower quality materials. They are EasyRest in Portland, Bed boss, and mLily which all have some better quality/value mattresses that are sold through retail stores but you would need to call them to find out if they have any retailers in your area and of course I wouldn’t know the knowledge or experience of any of the retailers that may sell them or the prices they may charge. I would also read post #2 here and post #6 here to make sure you are comfortable with some of the uncertainty involved with foam that is made in China.
Dixiefoam only has the one memory foam mattress listed on their site which is 8.5" (probably finishing at 9" with the cover) although they may be able to customize it to some degree. It’s more common to have thinner mattresses in lower budget ranges because of course more foam in the mattress costs more to produce (unless the foam is low quality and only there to create thickness and the “perception” of better value at the expense of material quality and durability). The base layer of polyfoam is very high quality/density and is 5.5" and the memory foam layer on top of this is 3" and is high quality memory foam.
They do make some thinner mattress that are just a single layer of latex or their high quality polyfoam but these have have no memory foam.
Phoenix