Tried searching for these locally at industrial fastener suppliers without any luck, even if they had them there would be a minimum purchase quantity in the 100's.
If you're handy, it's possible to make an acceptable DIY substitute at home... I used a 3/4" punch (OD), 1/2" punch (ID) and a 'Dahl' brand 2" copper plumbing testcap from HomeDepot. At 50 cents each, this provides enough material to make serveral washers. It won't have the same tension as the copper wave, but will serve as board protection from the retainer.
But if anyone locates a supplier for the original type, I'm also interested!