I've been using these guys for 15+ yrs:
http://www.wheelcollision.com/
They sell refinished wheels outright, on exchange, or will repair your wheel.
However, if you're looking for one slightly used that matches the patina of your existing wheels, craigslist, eBay, and specialty Porsche dismantlers such as Oklahoma Foreign are good sources. I've also used
http://www.car-part.com to search dismantlers nationwide.