I started buying my ammo at Bass Pro until everything became out of stock and it was take 6+ months to restock if ever came in. In the GTA I've lots of good things about Gagnon's Sports in Oshawa. There right next to 401 east bound.
Right now the cheapest place I've found to buy ammo for me is Ellwood Epps on hwy 11, when I'm going to the inlaws I'm alway stopping in there. I've saving anywhere from $5 to $10 on a box of ammo from them, then buying it down at Bass Pro or Lebarons.