There are a few places I go to for belts; fleabay,
sew classic, helen howes in the uk. I guess there are a few who specialize in parts for older machines. Do you know what kind of belt the orginal was? V-belts can be a bit hard to find in some sizes, but lugged belts and
stretchy rubber belts can work well. Stretchy belts are round and a bit more forgiving when it comes to size and adjustment. I would contact someone and describe your situation. I have to warn you, I have and old Singer, I've ordred belts three times now, and I still don't have the correct one. I might go fo a stretchy belt next time, and maybe try a lugged belt.