I got some similar balls at toys-r-us for $1 each (I can't seem to find them online now, however). They were more plastic-y than the rubbery originals. I had to remove one of the basket's counterweights because they were too light, but they worked great [see video showing the ball details]. I liked to run the game with only 3 of the 5 balls, as it provides less chance to jam-up the mixer motor and I felt the low # of balls penalized really bad shots by forcing the player to wait a bit for them to be shot again.
video (jump to 5:03):
toys-r-us product sheet: