I spent part of my childhood in central Mexico, and I can get a real nostalgia jones for tamales -- so I feel your pain. Montreal is not a great tamal town. The best luck I have had for Central American (Salvadoran) or Colombian style tamales is Supermarche los Andes (aka supermarche Gloria) at 4387 St-Laurent, just below Mount Royal.
For good Mexican style tamales, the only luck I have ever had is at Tortilleria Maya in Marche Jean Talon (on the Jean Talon street side). They don't always have tamales, but they are pretty good when they do (you have to ask, they are often hidden away). They are also my only source for fresh tortillas in Montreal. Call first: 514-495-0606.
(Part of the problem finding tamales in restos may be that for most Mexicans -- especially in central Mexico -- tamales are mainly breakfast food. They can be hard to find after 11am, Mexicans rarely think of them as lunch or dinner food. So in little restos that cater to a Mexican clientele, you may never find them at dinner.)