I had to have a custom mattress made because of the strange size and a corner being cut off so the slide would go in. I ordered it from an expensive Indiana company, Bradd & Hall, in Elkhart, but they messed up my order, and it was delayed.
However, the mattress tag says it was manufactured for:
Mobile Sleep Components
6732 West Coal Mine Avenue, #130
Littleton, CO 80123
But the above looks like they are marketing to dealers. It also says on that tag that it was made by WV11159, 627 Congaree Rd, Greenville, SC 29607. A search of that address shows it is a company called Engineered Sleep.
There were three levels of cost and I got the middle one and have been happy with it. The highest cost one was a little thicker and softer. I had the expensive Indiana company do the measuring so they were responsible for getting the size right, and they did.
Most importantly, you need a mattress that is intended to not need a foundation mattress.