There are a few good yarn stores here in Montreal. I recommend Mouline yarns, near the Atwater market (and subway station) http://moulineyarns.com/. There are very nice yarns, and you can be served in english without any problems. I also went to "A la tricoteuse" (779 Rachel East), they have very nice yarns but the lady there is VERY unfriendly and it's a bit pricy. I liked "A la lainerie Lepine" but it's far from downtown (http://www.lainerie.com/ - the website is in french). I also heard of a great place for cheap yarns, La bobineuse de laine, 2270 Mont-Royal East. They sell yarn by the pound, they can spool yarn for you, and do pretty much any yarn you want from what I heard. But I've never been there, strangely enough... :??
Hope it helps, and that you'll like Montreal!