I'd check alibaba if you're looking for anything in bulk really. Stuff tends to ship from China, but for rivets, who cares?
Looks like these guys have your size:
http://cncpart.en.alibaba.com/produc...ind_rivet.html
You'll have to get a quote, but it looks like they're no more than $0.005/piece with a minimum order of 10000. Be wary of methods of payment though.