The best way to get cash is to use an ATM. Of course, this does depend to some extent on the fees charged by your bank (hopefully none) but in general it's the cheapest and most convenient option.
Not all ATMs accept foreign cards, and the ATM fee varies from bank to bank. The most reliable choice is Woori bank, which usually has at least one "global" ATM at each branch, and only charges 3,000 won (much more than it should be, but amongst the lowest in Korea).
At Woori bank ATMs, to withdraw cash with a foreign card, select the "Foreign Language" option first: