We've made custom cushions from foam we got from https://www.thefoamfactory.com/ (no affiliation). They apparently do have "mattresses", but also just stock a bunch of foam that they can ship to you and you can cut to dimension.
We made basically a futon mattress out of wool several years ago. Has been holding up great. If you're not afraid to get creative, there's not really all that much to a bed. Especially if you get away from the coil spring kind.
There are plenty of foam suppliers in the US and probably every country, most of them will sell you rolls of whatever you want and plenty will let you come do it in the factory.
