'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
It's not a standard faucet washer. The OD is 0.803" and the valve hole is 0.158" initially (it gets bigger with use). The edge thickness is 0.162" but there is a raised circular "platform" 0.592" diameter and a conical projection in the center around the valve hole. Lowes or Home Depot aren't going to have these things and probably not a suitable substitute.
Most of the better mailorder/internet bike shops have them and here is one source: