I got reams of 120 and 220 backing paper from resently developed film in my paper recycling bag, so I could also make a slitter and get 70mm in the ULF but id prefer 120 film in 100 foot roll if the 120 film backing surface will survive my 220 pressure plates...

Most pro cameras post '65 (?) would accept 220 so a bulk reel of 61mm might even be a commercial option.