Both PetMart (the one behind the Costc)o and SuperPet ( the one behind the chinese buffet)don't sell cats and dogs. They only provide adoption.
You can go to Little Critters in CarlingWood shopping mall/Billings Bridge shopping mall / Merivale shopping mall. Just check it out in yellowpages, you will find its phone and location.
Also you can go to PetWorld in St-laurent shopping mall