1) Scour the frustratingly small selections at local bookstore chains. (However, these have improved from nothing in 2004 ... to a single shelf a few years ago ... to a genuine English book section with several book cases.)
2) Hit up some of the larger bookstores in Seoul with expanded selections.
3) Pay exorbitant international shipping rates through Amazon.com - something like $9 a book, with no discounts for multiple book orders.
4) Order from www.whatthebook.com. What The Book has a physical location on the side streets of Itaewon (near the fire station), where you can also buy used books. However, their real value is their online selection.
They can provide most English books in a wide variety of genres. True, they mark up the price a bit higher than Amazon and other US based online distributors. However, they don't charge for shipping in Korea if you spend more than 25,000 won.
Payment is relatively easy. You can pay through credit card or bank transfer. I do the transfers. Just jot down the info on a post-it note, and head to your closest ATM. In a few days or weeks, depending on whether they have your book currently in Korea, you'll be lounging on a couch with your new book.