There isn't really an IgoUgo category for a particular store that isn't a tourist attraction in and of itself so this will have to do.
Steve's Music Store, in downtown Montreal, is the epitome of music stores, the mecca for musicians looking to expand their stock of instruments and gear. Steve's covers most of a block on Rue San Antoine leading to the cross intersection of Rue St. Laurent and it lives in a few little shop units, most of which are not internally connected. You have to leave one to go into the next. A few are internally connected, mainly the guitars, both electric and acoustic.
Open since 1965, the selection is wide and varied in price. You can get pretty much any instrument you're looking for from strings to brass to drums and they also have turntables, PA systems, audio mixing desks and other gear for recording studios. They have a shop of sheet music though i'm told that's not the best of selections that you might find elsewhere. For musicians, it's like the "kid in the candy store" syndrome and you might be hard pressed to leave without spending a few centimes!
51 San Antoine Ouest (West). Look for the green and black storefronts.