I actually made my own shifter knob from a ten-ball some years ago to replace the little black ball.
I happened to already have a tap that was 3/8x24 (fine thread) and it matched the throttle stem, so I went and bought a single ball. Typical throttle that is on my '88 Tristar. Drilled shallow hole on a drill press and ran the tap in. If you have to buy the tap, probably cheaper to buy from the online site though. Only picture I could find that shows the throttle and ball. It has really yellowed in the ten years it has been on there.
1988 Tristar 190 Sport, 351W w/GT-40's, 1:1 Velvetdrive