Since the crisis in 2008 the Icelandic currency ISK (Icelandic Króna) has not been available outside Iceland. However we do have banks in Iceland and the first one would be at the Airport. It is open during all flights. Check the exchange rate here: www.arion.is
In Iceland you can use your credit card to pay for almost everything. Just make sure you change your pin code to 4 digits. Most self served gas stations have an older technology and cards with longer (more digits) pin codes will not work.