Any chance ball bearings from a piston head bearing spacer would work?

Easy.

Go to your local hobby shop and get a ball diff rebuild kit for a Team Associated RC18. Cost about 5$ and you get 8 bearings, and a little tube of grease as a bonus.

if you go to tokyo hobby's web site, they sell the baring for $3.00 and the rubber for $6.50 (difficult to navigate there site) but there part 14, and 16

http://www.tokyo-model.com.hk/ecshop/goods.php?id=2156

http://www.tokyo-model.com.hk/ecshop/goods.php?id=6107

damn... out $3 and a stamp! haha