I found a source for the hard-to-find (in the right length) piano hinge door bolts used on the 810/812 open cars. Snyder's Antique Auto Parts (Model A supplier) has them available. They come in a pack of 3, P/N 109-B. The current price is $1.50/per set of 3. Snyder's phone is: 888 262 5712
They are the exact size, shape head, and taper as the original, and may fit other applications like closed cars, but I don't know for sure.