I think the best deal is the 2 1/4" new ones from Ultimarc for 34 bucks or whatever it is. They don't have the spin of a 3" but at 1/4 of the price for new is REALLY good. Plus they are the same size as poolballs so you can replace the ball with an 8 ball (or whichever) any time in the future. Heck you can get some really really cool ones online (skulls sunk in clear whatever)
you can get some on ebay... but take note. I did, and didn't get even vaguely decent use out of it until I replaced the barrings for about 15 bucks or so. And even then its obviously not new.
Anyway, after that you want to get a oscar prehacked mouse for 10 bucks and a mounting plate (bob roberts is cheapest place I know... but for a few bucks more ultimarc and you don't deal with another vender...) your done....
btw, I odn't believe there is a single -non arcade- trackball worth adding to a cabinet. None feel right... and they aren't geared towards fast spins.