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There's a mom and pop hardware store around here (Washington DC) that has random washers, nuts, etc. that only cost a nickel each.

Not that I've ever gone in and tried to only buy a single washer. I'm guessing the old guy might just smile and say "it's free."

If you need a single washer and have a nickel, pick up your drill.

I don't think you are accounting for the inevitable trip to the emergency room.

Or galvanic corrosion.

In Japan, the 5¥ coins come pre-drilled for maximum convenience.