I find many used tires and rims by using kijiji. You can break it down by city. Look up auto parts and put in Porsche for search.
Here is an example in Edmonton
http://edmonton.kijiji.ca/c-cars-vehicles-auto-parts-tires-tires-rims-17-Rims-and-Tires-Porsche-Boxster-W0QQAdIdZ233860332