Bob Roberts has 'em for $6.00, but I'm sure your town has a local billiards shop or Sears... at either one, you should be able to find a cue ball cheaper than shipping a trackball. (Cue balls and 8 balls are usually available separately, while all the other balls are in sets.) Make sure it's a 2-1/4" cue ball, though; coin-op tables use a 2-1/8" cue ball, whereas regular tables use a 2-1/4".
--Chris