I wish I was looking for a 11/16-18, that size is very common. Unfortunately, many generator and alternator shafts take the oddball size 11/16-20. Thanks for the tip.
McMaster Carr (mcmaster.com) has 'em. Page 2525 and they cost $42.25. http://www.mcmaster.com/#standard-taps/=fl4wvt (Go to the thread size table at the lower left of the window.)
Good luck!