This is called an "apron" and was made by Kodak for their developing tanks. Ther is a metal plate with holes that goes over this to hold the film down. The manager of the darkroom deparment at my local store mentioned they were still available (at a ridiculously high price)a few years ago, so you could check at a store that might special order this.(posted 8399 days ago)Otherwise, check camera swap meets, ebay, used department at camera stores, garage sales, etc.
If you haven't tried them, though, Patterson makes the easiest loading plastic reels for 120 that I've used. I always seem to kink 120 with SS reels, but not so much on the Patterson reels.