Seriously, I haven't touched the prism on my GS-1(non-metering) since I put the WLF on about a year ago. (took the speed grip off the year before, too. It's so much lighter and easy to handle now...)
Sorry, I guess that doesn't answer your question, does it?
Ok... if you really want to know, here is a short-term solution. One of my Nikons was missing the eye cup when I got it. I cut a rubber disc from an old inner tube about 1 1/2", and cut a hole a little smaller than the diameter of the eye-piece attachment. When stretched to fit, it forms a convenient cone shape remarkably like an eyepiece cup. Not the perfect solution, but it did the trick until I found a suitable replacement.
In NY, try kohscamera.com- he often has Bronica parts, just didn't have the part I needed when I needed it. Alternative might be Dirk at japanexposures.com. He got me a new battery compartment door recently- it wasn't cheap (!), but if you need it, you need it, and it wasn't to be had in the US.