I am fixing a friend's old 1970s department store bike (Free Spirit ladies 10-speed). The left crank arm nut that holds the crank arm onto the crank stub is missing, and the arm falls off the square shaft. I believe the left crank arm is reverse threaded, right? I measured the outer diameter of the threads onto which the nut goes, and it measures approximately 10mm. I have looked around online and can't seem to find this nut on any online retailers. The crank is a Sugino, for what that's worth. Any suggestions where I could go to find this nut?