same problem. I have 2 mouse like your , who need a ball replacement. One of mine have a very degraded ball, the diameter is 33mm. It is flowen and clammy because of the degrade. Maybe the right measure is 32mm ? could be..
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http://www.ebay.it/itm/Baker-Ross-Palline-Rimbalzine-con-Amici-Marini-e-Brillantini-confezione-da-8-/390862338514
These kind of "magic balls" should work fine..