Using your ATM/debit card will generally give you the average exchange rate available. There is an ATM machine located within campus and two nearby the campus. Make sure to retrieve your bank card immediately after the transaction -- some ATM's have been to known to eat your bank card 15 seconds after the transaction (or one is forgetful!). It is also good to have a few travelers checks (VISA would be best) on reserve even though you plan to use your ATM card. Travelers checks can only be cashed with your passport at a bank.
Exchanging money before arriving in Costa Rica is not mandatory but be advised old 1,000 and 2,000 colones will not be accepted here. Be sure to check where you're exchanging is not carrying our old colones! Also keep in mind ATMs are better to receive the exchange rate than exchanging at the airport but also it is wise to have some money on hand.