Only two places: Chicago Pizza Kitchen in Vaughan, and Chicago Style Pizza Shack in Hamilton.
Oh, and if you're desperate, the Chicago deep dish pizza from President's Choice isn't half bad. But you're better off with one of the above.
Chicago Pizza Kitchen
2338 Major MacKenzie Dr, Maple, ON L6A3Y7, CA
I will spread the word because this place deserves it. Mickey's pizza in Clarkson has become a regular treat. These people know deep dish. I just had the bbq chicken deep dish dragon and it was OUT OF THIS WORLD. Sadly CPK in Maple is closed, however I would have never found out about Mickey's pizza in Clarkson otherwise. They make fantastic garlic bread with cheese too. Going to Mickey's has become a weekly event for me. Anyone looking for deep dish needs to try this place.