There are numerous sites that offer Porsche specific wheels . Even eBay can be a good resource to find new and used wheels at good prices. Provided, however, that you've been able to determine what the proper fitment for your vehicle is . Specifically, you need to take into consideration how wheel size (i.e. diameter and width), offset and tire size(s) will effect clearance .
This site seems to offer a lot of reasonably priced aftermarket wheels that are Porsche specific .
http://wheeldynamics.com/