FWIW, I installed a Rick Oleson screen in my 'Cord Vb, but had to return to the original screen because the focus was off. Rick offers a very nice and bright replacement screen, but be prepared that you may have to send the camera off, with screen, for a focus adjustment; or, perhaps, if you're a mechanical type you could fix yourself with shims. Anyway, I just wanted to pass this info on.
I bought a Mamiya screen from eBay for very little money (perhaps under $20. about 12 years ago). I believe it was for an RB67 camera (but not sure). I trimmed one edge to fit the Rolleicord with a razor knife. The focus was off similar to the post above from Alan9940 due to being a different thickness. I then sent it to a repair shop for proper adjustment. The screen is considerably brighter than the original.
I see someone mentioned igor camera - I have had several good experiences with him. The one return I had was not a hassle. He has several cameras that might interest you. Sometimes he takes a few days to respond to email -- then just call him.
I bought a Super bright Split Image screen by Jieying from China for my 3.5F - focus is spot on and it's significant brighter than the screen my Rollei comes with.
I also have the Oleson microprism - screen - no focus issues here with the 3.5F.
I have another Oleson (split image screen) for a Bronica ETRSI - it comes with a few shims and an instruction to fit them - working perfect for my needs.