That little plastic pin broke on my sister's 68 Valiant and I could not find one either. I bent a flat washer to match the curve of the shift tube and welded that on the tube. Then I welded a metal pin to the washer and that works fine. The first time I (almost) fixed it I welded the pin directly to the shift tube but it came loose so I added the washer. Surely it could not have come loose due to my crappy welding. In the unlikely event I ever need to remove the shift tube from the column I can just grind the weld off.
