I havent bothered to do the math to see what a 1:25 ball would be- I guess either 1/25 inch or 1/16 for a 1 and a half ball. You might check to see what the diameter is on those push pins that have round balls. I suspect they are a bit too big.
The next thing to try is to go to michaels or hobby lobby or other craft store and look through their stock of beads (from the beading section). Those things come in a lot of sizes, including balls. Neat thing is, they even have chrome balls! The rest of the hitch is just styrene strip stock.