You can find large ID, small cross section, O-rings at industrial supply houses. MSC and McMaster Carr are a couple of big ones that have extensive web catalogues and they are not expensive.
FWIW - These O-rings don't run very true. They are molded for a different purpose and without guides on drive and driven pulleys, will flip back and forth around the parting line. Polyurethane belting (round or flat) will make a better drive belt, but it comes on a spool, is cut to length, and then butt glued. Very hard to do without the tools and experience.
Hope this helps until you find the best solution
jim s.
