Outdoor and Camping Stores will have very comfortable mattresses used for camping and back packing that roll up tightly for storage yet self inflate when unrolled. You can add more air for a firmer sleep. They have other versions wth air pumps to adjust firmness.
And we actually found a re-upholstery shop in town that has a wide variety of foams of vaious firmnesses including dual firmness memory foam that he cut to size, for a very reasonable price. We even got toppers for our Home King bed and our kids beds too.
Getting comfortable on a sofa bed is kinda like the "Princess and the Pea"!
My posts shouldn't be taken for factual data, and are purely fictional, for entertainment purposes, should not be constituted as related to scientific, technical, engineering, legal, religious, spiritual, or practical advice. After all it's FREE! Amen. :W