http://www.jantzsupply.com/ has a large selection, and instructions on their use. Lots of good info on knife making.
But I don't use rivets on my knives. Instead I go to a hobby shop or hardware store and buy a piece of aluminum or brass rod. Cut a piece 1/8" longer then it needs to be, and set it with a firm blow from a hammer. File and sand flush. Sometimes I just glue in the rivets.
Understand that the rivets are only decorative. The knives are assembled with epoxy attaching the scales to the tang, i.e. the handle pieces to the blade. Be sure you use a waterproof glue.
YMMV,
txbob