There was recently a long discussion re: rivets on Yahoo Scampers, including some folks who have built airplanes -- the consensus was that the best place to get rivets is from the egg manf --
Scamp, for example, will sell an entire rerivet kit including rivets in proper quantities and sizes, acorn nuts, plastic caps and bases -- One needs to specifiy whether it has ratfur or elephant skin inside.
Alternatively, one can just call Scamp, describe the rivet location and they will sell a small quantity.