Since euros are the only form of cash currency that is accepted in Greece, this is the money in Greece that you need to use. It is typically better to only convert a small amount of money into euros prior to your trip and then withdraw more on arrival.
Where is the cheapest place to get euros in the US?
Though there may be a small fee if you exchange less than a certain amount, your bank or credit union will almost always be the cheapest place to exchange currency. You may be able to order currency at a branch location, by phone, or online to have it delivered to you or to pick up at a branch.
What is the cheapest way to buy euros in Greece?
The best way to get Euros in Greece is via an ATM or to exchange American dollars for Euros before you arrive. The exchange rate is more favorable, and the process is more convenient.