antique shops/furniture stores
Our '57 Overlander came with a fold-down back, free-standing couch. I'll bet that armed with your trusty tape-measure you can find something in a furniture store or antique shop that would suit your budget, taste, and size requirements.
Here's a photo of the Overlander's couch, unretouched, unrestored and untouched by human hands for years (including dust rags, vacuum cleaners, and any other cleaning implements...)
AIR 2053 Current: 2004 Airstream Interstate "B-Van" T1N Sprinter & 2006 Born Free 32 RQ Kodiak Chassis
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"