As a nerd, one of my favorite areas of Seoul is Yongsan. Yongsan station is easy to find and (pretty much) everything you would want is right there. A large CGV (with IMAX!), E-mart, I' Park Mall, and an e-Sports Stadium. Yes. E-sports. Because Starcraft is really a sport here. And one of the floors of the I'Park is half-dedicated to selling video games. You can find pretty much anything here you would regularly need, including games, systems and accessories. They even get fresh releases from Japan occasionally (nearly threw down $500 for a shiny Japanese 3DS several months before US release).