Hello Debby -- and a hearty welcome to the Forums!
I bought standard RV twins from Camping World and they fit in my Argosy just fine. Maybe shop with your measurements in hand but I'm suspecting you'll find what you want.
Sofa cushions? Do you have the original? The seat & back have fabric continued across the junction so that Junior won't slip between them if used for a bed. If so I would recommend taking them to an upholstery shop. We had them make up new foam and everything. Our '74 had an upholstered arm rest along one side. I disassembled that and took those pieces too. It turned out great -- except that it cost something over $400.
Hmm... now for the manual. The stock answer is, "If you are looking for manuals, you can get reproductions from Helen Davis at Secretarial Services, PO Box 484, Sidney, OH 45365 (937) 492-8885." Argosy was an Airstream product (see the orange badge next to your door!) but made at a different plant - Versailles, OH. It'd be interesting if Jackson Center maintains Argosy manuals. I do have my '74 manual and doubt there is much difference, but the spine is simple glued paper and I haven't had the heart to flatten it out on a scanner to reproduce it. Let me know if Airstream doesn't have it. I could set up to photograph my manual pages (same jpeg files as a scanner) and make it up on CD, but that'd be a winter project... If you're interested in more specific info or data right away (GVWR, tank capacities, etc), please let me know.
Have you seen Overlander64's Argosy brochures
. That is always interesting.