Here in the classifieds section is probably one of the best places to find OEM Boxster wheels for sale, I have got a couple of sets that way. Also you can check with Woody of Woody's Parts LLC (forum name itsnotanova) - I bought a set from him a while back as well. Check his thread for the latest cars he is parting out:
http://986forum.com/forums/boxster-parts-sale-wanted/52469-parting-out-whole-lot-boxsters.html
A lot of the 17" wheels he gets go to racers, but he often has 18"s, and probably also 17"s from time to time that don't go to racers.