A sheltered outdoor wall in the Seorak-dong Campground (a couple kilometers east of the main entrance to Seoraksan), it's convenient for visitors to the park (especially if you're camping here!) and residents of Sokcho.
Busses 7 and 7-1 stop at the campground. Camping has it's price, but the wall is free. It's just north of the campground store and hard to miss.
Water, bathrooms, cooking area and convenience store are all available at the campground, thought you'll have to pay a minimal cost to camp.