Acadia Mall (40E) and the mall close to green line bottom station have two biggest pet shops named Sah???, I don't remember the name. They sale $15-25/each. I saw them last weekend.
In the Code des Neiges Mall, there is a small pet shop in the base floor. I don't know if they still have them.
In Chinatown, there is sale during spring or summer time, not all the time.