I recently bought an NC-2 prism finder for my Hassleblad 500CM. The rubber ring (not the eye cup) around the eyepiece is partially coming out of its slot and is starting to fray. Does anyone know of a source to replace this ring? I already contacted Hassleblad/Bron and they said no.
A large, thin O ring from the local hardware emporium? With care(a lot) a clean install with contact cement works.
If you take the prism with, two O rings. One for the groove and one for the ring around the eyepiece gives a larger
surface to work with for the cement.
Try an local store that sells used cameras. They may not have the right eye cup for the NC-2, but they may have something that fits a lot of binocular rubber cups work. I picked up a shallower one that way to use with spectacles.