It's probably already to late to get a room for the Mud Festival (I'm assuming that's what you want to do). You can get train tickets in advance, but you'll need to be in Korea to do that. If you are already there, most large train stations have self-service ticket terminals (in English). When I went to the mud festival, my friends and I got train tickets and just made a day trip out of it. Bus tickets can be bought in advance, but they usually aren't unless its a major holiday. And even so, most buses don't have ticketed seats, so you might be able to cram into one on the morning of.
The Mud Festival is an insanely popular foreigner event, and all the accommodations at the beach are reserved far in advance.... they are also very expensive. When are you planning on being in Seoul?
http://english.visitkorea.or.kr/enu/SI/ ... cid=580007 is the tourism site, they are going to be your best source if you aren't in country to make your own travel plans.
If you do find transportation to get there, you might find a hotel in the city itself (which is a short bus ride away from the beach).