I can't help you as far as sourcing the washers however I would look at making them. You could make different thicknesses out of brass shim stock (sheets of brass that come in different thicknesses from very thin on up). I have an inexpensive punch set (use a hammer with them) and with a bit of care I expect you could make whatever you want. You could use aluminium from beer and pop tins but it would likely gall eventually (become sticky in use). Various industrial supply places should stock the brass and if not you can definitely get it through ebay.
If you use thin brass you could use scissors to make the outside of the washers but would likely need a punch to make the initial hole.
Best of Luck,
John.