Farm and ranch supply stores like Blains Farm and Fleet, Atwood's, Mills, Rural King, TSC, etc are good places to buy fasteners in bulk. Priced by the pound, they don't care if it's nuts, bolts or washers, if it's the same grade it's all the same price per pound. Our local Atwoods carries everything from grade 8 on down to grade 2 (no head markings), and a large selection of metric and the specialty stuff you find in the metal drawers and priced by the each or bag. They are also cheaper on things like gate hinges, angle brackets and the like.
We used to have a place in town that blew everybody else away, Shamrock Bolt and Screw. They closed maybe 3 years ago.