HOWEVER 120 backs used are REALLY cheap, you can get them for $20-$50 each on eBay or even Adorama at times, they have more expensive ones but the quality doesn't seem to differ, I own one that I got for my RZ67 and it's actually in better condition and looks brand new.
Deals are out there, good
You might be able to find 120 inserts but working 120 backs aren't giveaway priced. Keep in mind that the old Pro backs lack the double exposure lock-outs of the later Pro S backs. Both would probably be candidates for light seal replacement, too, along with the seals on the RB unit, especially on the surface that mates up with the film back.