I think I saw a number of sizes at Out Of Doors Mart when I was there three weeks ago. Give them a call, ask for the Parts Desk. They *know* Airstreams and will give you straight advice.
We replaced the original "brick" in the Queen BR with a wonderful TempurPedic just like we have at the Land Base. Sure, it hangs over the end of the short bed board bottom, but so what? We sleep solidly and wake up refreshed. Highly recommended. Worth every penny as you get "older" like us. Don't settle for look-alike substitutes or "toppers" as they don't perform like the real thing.