We've had a question or two about the balls.  They are standard softball-sized whiffle balls (12" circumference).  We got ours in bulk from EpicSports.com, but I would recommend our teams go to Dick's Sporting Goods or Dunham Sports in BG for the smaller quantity (and faster pickup).  Dick's has them in a yellowy-green, and Dunham's has them in assorted colors.