I found the actual screw-on knobs for the pull cables. They are the same as the ones used on the 63-64 Corvette. I bought 3 from YearOne, who had the best price I found at the time, and just got them in. I checked them against the only original I had, and I screwed one onto the pull cable, and they are IDENTICAL in fit, function, and looks! Here's where you can get them:
https://www.yearone.com/Product/1953-82-Corvette/FP273