Ebay (likely), www.classicindustries.com (maybe). I'm thinking these are the ones with the big round chrome face? (65/66 was the changeover year to the later small face ones but the innards still interchange). With a new cyl you can take it to a locksmith and match your ign key to it or if you have spares with the removeable rectangular chrome covers you can mix and match tumblers/spacers till any key will fit. The tabs on the chrome covers are brittle so bend them carefully just enough to get the cover off. You get the total height just right on all holes so the cyl will turn and even if you are short on parts and you leave several holes empty not to worry as no one picks locks anymore (exc in the movies)