There are 2 places I know of;
Fowler Interiors, 270-851-3257, email@example.com
James Ambrusso, 800-813-7686, firstname.lastname@example.org
I'm pretty sure except for the drapes/curtains of newer Airstreams they are made to order.
To remove your drapes, at either end of the lower and upper tracks there will be either a small pop-rivet or screw, since yours are original a pop-rivet will likely be there, drill out the rivet and you can slide out the curtains. When you replace the curtains use a (sheetmetal) screw in place of the rivet so its easier to remove next time.
For the clips, they are still available through rv dealers or when you contact one of the places above ask them.