Exchanging $$ for Euros

http://www.ricksteves.com/travel-tips/money/cash-tips

http://www.investopedia.com/articles/forex/06/interbank.asp
Interbank rates are the wholesale large transaction lowest bid-ask spread rates.

http://www.investopedia.com/articles/forex/090914/understanding-spread-retail-currency-exchange-rates.asp

The bid-ask spread is simply the difference between the price at which
a dealer will buy a currency and the price at which the dealer will
sell a currency. In other words, the bid price is the price that the
dealer is willing to pay or “bid” for a currency, while the “ask”
price is the price that the dealer wants for a currency.

Consider an American traveler – let’s call her Ellen – who is visiting
Europe and is confronted by the following price for euros at an
airport exchange kiosk: EUR 1 = USD 1.30 / USD 1.40. The higher price,
i.e. USD 1.40 is the price that the dealer is asking per euro. Since
Ellen wants to buy EUR 5,000, she would have to pay the dealer $7,000.

What if the next traveler in line – Clark – has just finished his
European vacation and before boarding his flight back to the U.S.,
wants to sell the euros he has left over. By sheer coincidence, Clark
has EUR 5,000 to sell. He would sell the euros to the kiosk dealer at
the bid price of USD 1.30, and would receive $6,500 for his euros.

The difference of $500 (i.e. $7,000 – $6,500) arising from these two
transactions represents the kiosk dealer’s profit, and arises from the
bid-ask spread.

Best exchange rates, generally, using a bank ATM. Let your bank know beforehand that you'll be using an ATM card in Greece on a trip. I usually have found convenient bank - not private -ATM's in any major airport after arrival.

If you have an ATM Debit Visa/Mastercard from a bank that waives transaction fees you'll get the best exchange rate in Greece. Greek banks don't charge fees. The two cards I use when I travel that don't charge fees are Capital One and Charles Schwab.

We always wait until we arrive at the airport and go to the ATM(s) there and withdraw money. Exchanging $ for euros in America is in most cases a rip off where you get charged an outrageous exchange rate, even at your own bank. As stated above let your bank and credit card company know you will be in Greece and the dates. By the way if you have a credit card that charges foreign transaction fees get another one that doesn't.

Today's rate is about $1.11 to the euro. It's been below $1.10 in recent months, but over the last several years it's been in the $1.30+ range. It changes daily, but not very quickly. As others have noted, your best bet is a bank ATM in Athens, using your debit card (after having notified your home bank about your travel plans). You might want to use an airport-based currency exchange kiosk if you need a few euros as soon as you land, but you will pay more for them.

I look for ATMs at staffed bank branches in case the machine eats my card or I need to break large bills. Sometimes they're inside the branch, or in an enclosed lobby, which is safer.