The cheapest option is generally to use a bank card at an ATM in countries that use the euro or by making purchases there with a credit card. If it is necessary to buy traveler's checks or cash, the exchange rate is generally better outside the United States.
When buying foreign currency before traveling, banks are a reliable source, but a thorough Internet search yields the best results. Visit a site such as XE.com to see what the current exchange rates are, and compare these rates to online currency dealers before buying online. Since the profit margins are small, pay close attention to the details of such purchases. It is not uncommon for unscrupulous dealers to build extra costs into these sales.