You can purchase it on the Korean Air Korea website. You will have to download a bunch of plug ins in true Korean style. You can also use foreign credit cards. You don't have to go to the office, you just show the credit card used at check in.

It's a common practice for many Asian airlines. Vietnam Airlines required this, except the credit card used was to expire by the time of flight. I scanned a copy of the front and back and emailed it to a ticket office. It's an easy requirement to get around.