How thin does it need to be? Could you use sanded plexiglass?
Cutting down a bigger ground glass is a good idea if it is thin enough. Take it to a window shop to get it cut. They can probably cut it neatly and grind the edges for a few bucks.
What do the folks who replace the original screen with Beattie or Maxwell screens do with their old ones? I'd think there'd be some around. In fact, when I get mine replaced I'll give you my old screen; I just don't know when that will be.
Also look for "parts only" Rollei's. You might be able to get one with a screen for cheap.
Do you know what kind of Rollei TLR you have? I recently aquired a Rolleiflex 3.5E with a Zeiss Planar. I think it was made in 1957. I've been shooting it for about a month now. I LOVE IT!!! I love looking at a ground glass instead of squinting through a viewfinder. I also really like the ease of getting low angles. TLRs may be the perfect camera for taking little kid pics. Recently I've been blowing up some Rollei shots to 16" x 16". They look awesome.