[lupin..the..3rd]

I've seen the 18" five-spoke (sorry, don't know the actual name) 997 wheels on a '99 996 and they fit just fine, no spacers or anything. Any wheels from a narrow body 996 or 997 should fit your cayman. C4S, turbo, GT2, etc. wheels will not fit; too wide and wrong offset.

You could always find a local seller on craigs list, bring your floor jack, and test fit them before you buy.