ATMs are common in Sri Lanka, and most of them work with Visa (Plus), MasterCard (Cirrus and Maestro) and even American Express.
ATMs often offer much better rates than exchange bureaus, as long as you choose to be charged in the local currency. Withdrawals in Sri Lankan Rupees are calculated using the mid-market rate.
While using ATMs is the easiest and most convenient way to get the best exchange rate possible, you do need to watch out for fees.
ATM operators in Sri Lanka charge a fee for every transaction. This is usually between 200 LKR and 300 LKR. Your home bank probably charges fees too. You can usually expect to be charged an ATM access fee and a foreign exchange transaction fee.