Since banh mi is influenced by the french baguette maybe look for a "boulangerie française" that make a great baguette. I suggest
1291 Avenue Van Horne
Outremont, QC H2V 1K5
3660 rue St Denis
Montréal QC H2X 3L7
(croisement St Denis et Cherrier)
Could you just buy the baguette from your favourite banh mi place?
Else, in Verdun, Marche InterAsia sell baguettes for banh mi. Tthey also sell their own version of banh mi, which is ok, but not the best - edited to add, it is only ok because of the fillings used, not because of the bread, the bread is good!