I was going to suggest Lebaron as well. They don't exactly have the best customer service, but the prices are pretty good.
Believe it or not, there are a few Canadian Tire stores in Scarborough that sell ammo. The one at Markham and Lawrence in Cederbrae Mall even had handgun ammo the last time I was there. Prices however are elevated.
I say believe it or not because you normally can't even find BB's or pellets for airguns in CT stores in Toronto.
Personally I wouldn't buy ammo from CT because I get the impression they don't take very good care of the "book" that they record your info in when you buy ammo. Too many punks working there that could have access to it. My uncle told me one time he was in a CT store, the book was lying open on the counter. He looked through it for 5 minutes, and not one employee stopped him. He asked to speak to the manager, and ripped him a new one.