+ 1 to tires23, they usually have the best prices.....
You can also try Exclusive Wheel and Tire (in Ajax).....they're kinda new and are trying to make a name for themselves and offer some REALLY cheap prices on some stuff. I got a set of Falken Fk 452s for a good few hundred bucks cheaper than anyone else in the GTA including Tires23....check'em out,
www.exclusivewheel.com