On my Double-Up, if the shutter was installed incorrectly (one clip installed above the shutter board), it would push down the shutter for one hole - did not impact performance from a player's perspective, but if the ball landed in that hole (in my case #1), it would sit slightly lower, and the ball would magnetize.
Check your shutter boards carefully and make sure they are fully clipped in. I kind of doubt this is your issue, but you never know.
The games originally came with non-magnetic balls - they are more expensive than regular balls, but this should prevent the issues. As @mrbally says, the Pinball Resource has what you need.