my friend Clint O'Conner has designed an adapter that allows the use of 135 film in cameras that take 120 film.
Of course, you lose the ability to accurately index your frames, and should, in fact, tape the sliding cover for the frame number window in place to avoid ruining your photographs. I get that 135 film might be easier to source and process sometimes, but with the Internet, you can buy film online and send it to remote labs for processing. I promise you that 120 film makes better pinhole than 135, by virtue of the larger frame size.
Hope that helps.