Ed,
Not everyone has them, but most hardware stores that have bins with different size screws, washers, etc are usually your best bet. These washers also work well with nylon wing bolts to spread the load so maybe micro fasteners would have them as well (or anyone who sells that type of item can order them for you). On a side note I've used them on both knurled and smooth back plates and they hold up well......even in high RPM applications.
Good luck,
Jim