If you have a specific rim in mind, keep an eye out on eBay, Craigslist, 986forum, RennTech, PCA, etc If you don't have a specific rim in mind, also keep an eye out until you see something you like. You should be able to buy almost any set of used OEM rims for sub-$1000. I would stick with a 17" rim, especially if you plan to do any track or autocross. I paid $600 + shipping for my set of 911 10-spokes from eBay.
Not OEM, but you can see all of the styles:
Porsche Wheels from Wheel Enhancement