You can use rare earth (neodymium) magnets but they need to be on the smaller side. They give a clear strong tone. The problem I see with the balls is how would you orient the magnetic poles? I would stick with (no pun intended ha ha) discs, bars, etc. - something you can orient.
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You can get small rod rare earth magnets from Lee Valley.
http://www.leevalley.com/en/wood/page.aspx?p=32065&cat=1,42363,...
You can use rare earth (neodymium) magnets but they need to be on the smaller side. They give a clear strong tone. The problem I see with the balls is how would you orient the magnetic poles? I would stick with (no pun intended ha ha) discs, bars, etc. - something you can orient.