I think a lot of foreigners use Xanandu Travel
in Itaewon - they cater to foreigners as their agents all speak English. You can make all your arrangements over the phone/internet and they'll just mail you the ticket, wherever you are in Korea. They've been ok in my experience, no major problems.
The great thing about going to a travel agent in Iteawon is they actually know what travel insurence is. As foreigners we're not allowed to buy the travel insurence at the airport