Hi, wr630,
Whatever, you do, do not get money from one of those exchange booths (like Travelex) at the airport. You will definitely lose out on those transactions. As others have pointed out, the express bus in to town takes credit cards, so best to get to an ATM near where you are staying in town.
Look for an ATM at one of the major banks - Clydesdale, Bank of Scotland, Royal Bank of Scotland, Santander, etc. None of these banks will charge you for a withdrawal using your US based debit card. Any charge would be initiated by your home bank. Check with your bank to find out if there is a charge for foreign ATM withdrawals. If there is, you may want to make as few withdrawals as possible while in Scotland. Some US banks will waive foreign ATM fees if you keep a certain amount of money on deposit. Some of those banks will limit you to a certain number of ATM transactions in a certain time period before they start charging you.
All of the banks in Scotland use the same exchange rate, which of course will vary from day to day. So one day you may withdraw 100 pounds and see that you have been charged $120.00, and the next day you may withdraw another 100 pounds and see that you have been billed $121.50.
As with anywhere else, be aware of your surroundings when using an outdoor ATM.
Best wishes for your holiday!
Mike (Auchterless)