Most banks have a currency exchange rate on their web site. If you pay with a credit card you will pay the rate of exchange the day the amount is processed (that is what I understand).
On the other hand, even when your c.c. charges a fee for the conversion to U.S. funds, you can sometimes wind up ahead, it all depends on the value of the dollar at the time. When I paid off a cruise on my c.c., the Cdn. $ was high compared to the US $. Hey, presto -- I "made" $150 on the deal!