I run bridgestone blizzak LM-60 tires mounted on the cheapest 18's tirerack had. 245/45/18 square setup. No spacers. Spacers are just for aesthetics. Here's a link to a few pics:
winter tires top was from last winter (without my FI exhaust
) bottom was from a few weeks ago. You can go to tirerack.com and select make and model, then press the winter tab and it will spit out a bunch of winter tires that work for the z.