I have a Radio Receptor UHF converter that I purchased NIB. You can use either channel 5 or 6.
http://antiqueradio.org/RadioReceptor.htm
I haven't used it regularly, since it has all the original caps and the UHF stations in this neighborhood are uniformly awful. However, I have tried it out with my Predicta and my RCA 630TS and it seems to work just fine.
You could try sending an email to Mark Nelson. His website shows his collection of something like 100 converters and boosters.
http://www.geocities.com/k8zhd/TVBox.html
According to Mark, the RR converter was sold under several different brand names, including Silvertone, DeWald, and Truetone, so it might be fairly easy to find.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html