The Stovebolt Site Forums The Shop Area Paint and Body Shop *Rear Fender Rivets*

Here's a good presentation of tubular or hollow rivets that are used for the rear fender brace.
Tubular Rivets
These guys probably sell in minimum lots of 1000 (I haven't asked). They should also be available at the larger hardware stores. Measure the thickness of the fender and brace and the shank diameter and add the clinch allowance. You'd need a setting tool as well.