Some towns have stores that only sell Nuts and Bolts. Yes it sounds crazy but these places do exist. If all else fails, you could try a machine shop. If you can find a Thread pitch gauge, get one, they cost $10 or $20
73super, I have a spare nut from a 60s truck and it appears to be a 7/16-20 Jam nut. You should be able to find that at your hardware store in the National Fine section. Hope that helps.