The original 1mm spacer that Shimano supplies is superior to the 1mm spacers that most suppliers have. Try and stay away from the aluminum 1mm spacers. The aluminum spacers tend to bend/warp and can even break under the pressure of tightening the lockring. Shimano uses a spacer made from steel and will last many years -- even after the casette is shot the spacer will be good. So most shops should have at 1 or 2 sitting around.