I have a very old Donkee ball spinner. The threaded rod that feeds from the under side of the ball cup to the bottom of the spinner where the large belt pulley attaches has gotten pretty worn from over time. The "flat spot" where the set screw clamps down is worn enough that I constantly have to flip it over and try to tighten the heck out of it in an attempt to keep it from slipping. Any idea on where to find a replacement or any kind of fix to attempt?

It might not be the best view assembled but maybe you can see where the top of where the flat edge on the threaded rod is worn. If I had to guess, it's molded into the ball cup.

As a temporary fix, you could try adding a small shim or washer under the set screw to take up the slack in the worn flat spot. This may provide enough grip to keep it from slipping.