Most tourist area stores in Canada will take your US$ and give you Can$ as change. Some post a sign with the rate they are using. Canadian Banks will also change currency. At some border crossings there is a "Money Changing" Business located quite close to the road, they probably don't offer the best rate in town though. There are cash machines in many places and Mastercard/Visa will do the exchange thing as well as someone said above.
Money talks, have a good trip, JohnB