There are lots of "custom matteress makers" out there, any of which can do the custom shaped corners for an Airstream.
We found one here locally (Boulder, CO) we plan on using when the time comes for our '56 Safari. It's called The Foam Source
and they will make a custom mattress out of any combination of foams you want - latex ($$$$), posturpedic ($$$), polyurethane ($$), other synthetics ($) - you name it they have it all different thicknesses, firmness levels, etc...any shape you want - give them a template, pick your materials and in about 3-4 weeks you have your mattress. They also have their most popular mattress compositions made up so you can kick the tires a bit too. Their prices weren't too bad - no, they aren't gonna give them away, but at a slight premium over a standard, ready-made mattress, it's definitely worth it, especially if you full-time!
They even have "free shipping", but then you can't kick the tires....I'm sure there is a place nearby you that's similar, get out those yellow pages ~