Our queen sized mattress was custom made by the Verlo company, and was an option the teardrop maker (TCTeardrops) offered---cost $250. I've noticed the price has gone up ($264), since we bought ours. Very well made, and extremely comfy, yet malleable enough to fold into a u-shape. Takes both of us to maneuver it out the door though. Will be easy to occasionally turn it over (to even out the wear) and air it out, before and after camping seasons.
There's Verlo outlets/manufacturing centers around the country, and I believe they'll make and ship mattresses of any size or shape needed. Then there's the option of contacting Todd at TCT, perhaps he could sell/ship one through his company (located in Wausau, Wisconsin).
Here's a photo of mine, before I unwrapped it and sewed up a fitted cover for it.
