Inland RV carries all sorts of OEM parts here's the link:www.inlandrv.com
Call their 800 number for a more hands on experience.
I think that seals and gaskets are still available.
The beltline vinyl insert trim (mid-way up the body) on Argosys is a problem..... Orange is no longer avaialbe in the narrower widths but blue is. Blue was used on latter Argosys - 78 and on. You might find some orange languishing on the back shelves of an airstream dealer if you dig. Be sure you know what width you need - the lower trim is wider.