How do i pay for a flight in USD on a Canadian credit card?

I am trying to find cheap airfare flying out of toronto, but the sites keep showing me USD. I'm in Canada. How can I pay the fares with my Canadian Credit card?|||The card will convert the currency.|||Many sites actually have choice of currency. I am in UK so websites I go on automatically show the price in British pounds, but in some situations it is cheaper for me to pay in American Dollars - so I change the currency on the website