I went to an upholstery supply warehouse for a foam mattress. They let me test-lie on all different thicknesses and densities of foam, so I could choose the one my back liked best. And it was far less expensive than buying retail!
PS: This was in a town near a major RV manufacturer, and I think that contributed to the large range of foam options. I tried to replace my mattress when on the road, in a huge metro area, and found options more limited. Eventually got something close to what I wanted, but it took a lot of legwork.