Just a note, if anyone else has the same issue: I managed to replace the viewfinder, and it was not really that hard. But, I'm not sure I'd do it again. I don't feel very comfortable fixing this stuff myself, the looks of things when you open up the camera is rather overwhelming and it's apparent how much electronics is in these things. (I have built a desktop computer and replaced some RAM, etc. before, but that is my only knowledge with electronics.)
I bought the part from the local Pentax distributor. It is really a Pentax spare part, but they are identical. The bag says 76830 - OM301, which I guess is some kind of identifying number for the part.
All sides of the camera needs opening before you get two the two screws holding the viewfinder in place. The most difficult step was removing some wires so that the top and back cover could be put aside.
The service manual has some nice instructions. Find it here, together with other related service manuals:
Downloads
Anyway, my viewfinder is replaced, it cost me $50, and took 20 minutes. I was told fixing it at a repair shop would cost $300-350
Hope this comes in handy for anyone else having the same problem.