I started a mini-quest tonight to see if I could find any interesting washers, and wandered into the realm of bead caps, like these (scroll down about halfway for the shallower ones). If you navigate back to Findings, then check under different metal types, the site has them in sterling and copper, then under 14k gold they have something called Rondell Spacers.

Has anyone used any of these things as washers for pinning?