and  to Aussie Pinball !

An interesting problem. I've heard of NEW IN BOX Stern owners replacing the factory balls after a week. But this is not due to rust ! Sounds like you were sold second hand balls - which is common unfortunately.

I buy from RTBB ;

http://www.rtbb.com.au/catalog/product_info.php?cPath=140&products_id=2956

These are very good for all pinball machines EXCEPT games with magnets, as these balls cause havoc when magnetized ! But they retain their shine, and they won't be rusted when you receive them !