Me, I would start at the auto parts. Not one of the chains, but find a real auto parts. One with grease on the counters. Where the mechanics go. They may be able to find one.
If you have a number on it that would really help!
My next choice would be someone like Motion Industries web site. They will deal with individuals. But you have to do some homework and know some details about the belt sizes. But odds are good you can get the belt from them.
Sears would be my last choice because they will much higher in price than one of to the other choices. And odds are it's a standard if not common belt.
Jeff