I have just bought some copper rivets from this British supplier, who will send them domestically for £1, or to most other countries for £2.50. They have various sizes, and I bought brass as well, and although they don't seem to have those on sale now, they might soon.

1/16" x 1/2" copper rivets for model making qty 50 - eBay (item 360235404662 end time Aug-09-11 08:10:18 PDT)

I also have some brass and stainless tubing (a rarer item, from eBay a long time ago) with 1/8 diameter and 1/16 bore. It would be usable for cutting a lifetime's worth of washers (doming them first with abrasive paper if required) or pivot spacers. It is a good but sliding fit on the rivets, but a tight driving one on K&S 1/16 brass rod.

You can also form domed washers in a simple die. You can make this by drilling a 1/16in. hole in a piece of steel, and countersinking it with a round-headed burr. It will last quite a while on brass or copper without hardening. If you are prepared to risk breaking the burr, you can impact it into the bottom of the countersink with a hammer, making a die which will imprint an interesting pattern on the rivet-head or washer.