You know, Retro is really popular right now. Don't limit yourself to checking just for airstream sofas. Look for other sofas on craigslist and your local resale shops and retro stores. We even have a retro store in St. Louis that has shelving full of that fun crazy fabric.
Another source may be etsy.com. There is a lot of retro fabrics on there, make sure you are searching in the vintage section.
I bet you will be surprised and find it, it's hard to be patient, but I think you'll be successful!
Tina and Mike
1959 Avion e-20