The only place rivets might be is to rivet between the rim and the center. If they have to be replaced, I would drill out the holes to a standard size hole and order some steel rivets from someone like these guys.
http://www.bylerrivet.com/products/solid-rivets.aspx?cat=41

I am in the trucking business and we set rivets all the time. A semi trailer repair shop should be able to buck the rivets for you. They probably wouldn't have steel rivets on hand. Only aluminum. But you would want steel.