In many countries you will get your best rate at the airport - not in Costa Rica. They gave me a rate about 15% below the going exchange rate - knowing that I had just arrived and didn't know better. It might not be necessary to change money at all. Almost anyone will take dollars and will give you the normal rate. ATM's also offer a better rate. I got the best exchange at Banco Nacional ATM and wasn't even charged an ATM fee.