Can you give a little bit more information? I think there is help for you with the sander. Can you find a model number or a make of any kind. I have seen belts made for sanders with what we used to call alligator clamps. they pressed in to the sand paper and clamped underneath. We used a what we called a breath tape at times. I don't really know the actual name of the tape. We called it breath tape, as we hold our breath hoping it was going to hold together. But it worked pretty good.
I would think somewhere there are some identifying marks or numbers.
Sorry, I cannot be of more help. One other thing I saw an old wood worker do, now that I think of it. He took his belt and slid over and around the sander frame. He then laid his belt on the table and measured the sanding material to the exact size of his belt, minus a 1/32 of and inch. it worked perfectly. You may try it. The belt thickness is of course thicker than the sand paper. That is why he cut it a might shorter.

Good luck.