Welcome to Canada. I’m an UK Expat of 37 years.
There are a number of resources for selecting winter tires & rims. My first suggestion is to go to http://www.tirerack.com There is a wealth of information on this site and you can enter your vehicles information to see what is recommended for winter tires & rims. Whether you buy from The Tire Rack is up to you, but it gives you the information of what to look for locally.
Next up is your local Porsche Dealer. Probably not the least-cost solution, but they do sell winter tire/rim packages and the rims will be Porsche style and quality. Also check out the Tire shops that advertise in Provinz. These guys have plenty of experience with Porsches and can guide you to the best solution for your C4S.