I used to have a Nikon F4s and I did managed to find the MB-20 grip one time by accident during my first trip to Japan. I got it for USD5. It was in one of those snazzy Japanese camera collecter ships in Akiba. Really made the system heaps lighter though you loose some fps.
Sorry just wanted to voice out how modular and great the Nikon F4s system was (sold it off to finance film purchases hahah), I agree, after I bought that piece, a lot of people were commenting also on how hard it was just finding the component alone and how lucky I was to have found it in the first place.
Anyway, good luck finding it, I tried to look for the split prism focusing screen when i still have it but never found it, I may have kept the F4 had I found that screen.