If it's a copper crush washer, you can get them at Lowe's or Home Depot in the plumbing section. If it's an aluminum washer, I've block sanded them with 400 grit, and re-used them (as long as they weren't distorted to start out with) and never had any problems.

Edit: My local Pep Boy's has them on the aisle with the HELP products. Most auto parts stores should carry them, too.