I make my own pool ball knobs.
Pick up a set of used balls on ebay, I got mine, sadly minus the 8 ball for under $20. I used the one ball 'cause I gave the three ball and the 14 ball that would match my green jeep to other people. The cue ball is in my Cummins pickup and the four ball is in my LandCruiser.
Figure out what size tap you need, go to the hardware store, price one and put it back on the shelf. The things are friggin' expensive!
Go to the nuts 'n bolts aisle and get the proper size bolt. Grade 5 or 8.8 will do--pool balls are pretty soft. Go home and haul out your dremel tool and cut the bolt into a tap. (see pics)
Drill a hole a bit undersize for the bolt, use the bolt to tap it, mount.