If the door gasket is the older rivet-on style, gasket material is available at:
www.airstreamdreams.com
Inland RV sells door gasket material for the years '58-'63. I bought some, and it is very "generic"--basically just a plain square gasket, about a half-inch square. This might work for your '57.
www.inlandrv.com
As to the rest of your questions, the guy I'd ask is R.J. Dial. He has been restoring a '56 or '57 Flying Cloud. You can see photos of the restoration at his website:
www.vintageairstream.com
R.J. may have some spare parts, or he can tell you where/if the parts are available. He can be contacted at
webmaster@airstream.org
Good Luck!