Check the rules on your credit card or atm card. My preference is to withdraw money from my bank account using my atm card at a Canadian BANK's atm machine (stay away from the no brand atm in bars and stores). Most of the canadian banks do not charge extra for the service, although a few do. Your own bank probably will add a service charge, but it seems to still be better than that charged by the foreign exchange places in Montreal.
Try the Scotia Bank, Bank of Montreal, TD, or Desjardins machines, they are easy to find, and do not add service fees.
Credit cards work good too (for purchases, not cash advances), and usually give good exchange rates, but make sure your issuer is not adding extra fees. Of course, there are the usual downfalls of using credit cards: easy to spend the money like you actually have it!
In my experience the Canadian banks downtown will not give you a better exchange rate than many of the currency exchange places in the area. And do not rely on the rates posted on the boards in the window, as they do not reflect the added service charge, which will vary from place to place. All of them have them, including banks. So it makes sense to exchange all you need in one transaction,(and pool together with your friends) to minimize the fees.