There are four official Lego stores in Korea and here's the full list.
Lotte World Mall Jamsil B1.
Pangyo Hyundai Department Store 5th Fl.
Starfield Mall Goyang
Busan Seomyeon Lotte Department Store 6th Fl.
Pick a brick lego, you can buy individual pieces at the shop. All the stores have a Build-A-Minifigure zone.