It's easy to find foreign bank atms such as Citibank and HSBC around Seoul. Otherwise you can go into a convenient store and withdraw cash from ATM inside, as most convenient store atms are capable of global transactions.

You shouldn't have too much trouble finding an ATM that does global transactions at Incheon airport. From what i can remember, even the local bank atms did global transactions at the airport.

If you plan to stay in Seoul for a few days, you should def get a T-money card. you can use it for public transports, taxi fares as well as even pay for things at fast-food places and 7-11.