You may want to post a picture of the piece you are looking for.
I bought a Hehr rigid plastic screw cover off amazon a couple of years ago. (I don't think they have it in black anymore.) They're the pieces that frame the perimiter of the window and cover the screws that hold the window to the body.
Vintage Trailer Supply has the type I bought in a 4 foot lenght.
Look at: Fixing Leaks - Windows & Vent - Boler-Camping
for a description of some of the other seals.