Cheap groceries in Quebec City!
There's not many cheap grocery stores in Quebec. You can try the big supermarket such as Maxi or Super C. If you like more ethnic food, the small asian/latino american/african grocery stores in St-Roch or St-Sauveur have some products at a cheaper price than regular grocery store (and some more expensive).
If you really need to save money, dollar store often have food (canned or dry) but the quality is not often there. "Les escomptes Lecompte" is a popular "dollar store" in Quebec city.