Depending on where you live, Mantis Racing in Oakville can help you out. They`ve been in business for over 30 years, a respected UCR supporter and have outfitted many a Porsche for winter.
I personally like Continental Tires and their Extreme Contact DWS is a great Fall/Winter/Spring tire depending on your driving habits.
What is important is where you will be doing most of your driving this winter, QEW to downtown Toronto or north every wknd to Collingwood skiing, it makes a difference.
Andy is bang on in his recommendations as a start. Do your homework and call around for prices now.
There is always limited production in these larger 18″ tires and they sell out quickly.
Good luck with your search and welcome to Canadian winter ! – Wayne