I am fortunate enough to live in an area that has a Mennonite and an Amish community. I purchased an old treadle machine at an antique mall. It had a leather belt that was broken. My local Bernina dealer is a Mennonite. He was able to provide me with a new belt.
If you have that type of community within driving distance from you I would check with them. There is probably some place on line you could also purchase one.