Don't know if this will help anybody, but my 5/8 SS washer search finally came to an end!
I was at Home Depot and after looking through the drawers for about 30 minutes with no luck, out of the corner of my eye, I spotted a "bolt kit" hanging on a rack. It's in a small plastic package with an orange label and has four SS 5/8 nuts, fender washers and lock washers. It think it was about $4.
Edit: Now that I'm home, here's some more info: It's called "Create-A-Bolt" by Crown Bolt Inc. Product number 00692.