Many butcher shops and grocery stores in Quebec City tend to carry lean meat owing to current public demand—you can blame the dietary guidelines for that. Fattier cuts are harder to come by. However medium (mi-maigre, in French) ground beef is a reliable option.
Costco sells AAA ribeye and striploin steaks that are far cheaper than elsewhere. They also have pork belly.
Chuck steak (palette de boeuf, in French) at Boucherie Eumatimi is usually well-marbelled at Prime grade while being cheaper, and is delicious when cooked sous-vide 24 hours. It needs spices, mainly pepper and garlic.
If beef is not your thing, then Marché Label Terre sells lamb belly that is supremely fatty. If you ask, they will provide harder to come by organ meats, like spleen and brain.
Fatworks is a food company in the United States of America, so you will need to order online and pay customs fees. They sell organic 100% grass-fed tallow which I find to be a perfect fat to cook meat with (it imparts flavour when stir-frying ground beef).
For eating out, the only restaurant I can recommend is Phil smoked meat. Delicious smoked meat, but be wary of what else they add to it. Last I checked, it was a variety of spices with small amounts of sugar. The waitresses are not always well informed, so ask the owner directly as to what else goes in the meat.