I buy rivets from these two sources:

http://www.mcmaster.com
http://www.bigflatsrivet.com

McMaster has a very good selection and good prices.
Big Flats Rivet Co. has a better selection and higher prices.

I've seen these folks at a trade show, looks like they have a good selection of all sorts of rivets and rivet setting tools. Expect I'll give them a try next.
http://www.bylerrivet.com/

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-