As mentioned previously by OBD, you have to check out Bass Pro, located in the Vaughan Mills Mall, just north of Toronto off Hwy 400 (south of Canada's Wonderland). It is a typical US-style "big box" store that sells just about anything related to fishing, hunting, camping, boating, etc. In the store, they even have a real float plane that is suspended from the ceiling along with a huge aquarium loaded with live fresh water fish including some real hogs (largemouth bass from Florida). Large selection of tackle but not the best prices...
For the best prices, you should check out LeBaron's. They have 2 locations just outside of Toronto, one in Mississauga and the other in Markham. You will be surprised at the low prices they have. In fact, I just picked up a bunch of stuff today for my fishing buddy in Richmond. I'm fortunate to be able to get out to Vancouver quite frequently and get standing orders to bring tackle out with me. Perhaps a good business opportunity for me...
I realize that you were really looking for a store close by downtown Toronto but there really aren't any to recommend. If you have a car, it is quite easy to get to the places I mentioned.
Happy shopping!