There are a number of bike shops in Seoul that are relatively well equipped. Outside of Seoul, Pusan and Taegu, the situation isn't too great.
Good maps are at the airport upon arrival.
I only know the Seoul scene. There are some others, but these are closest to my riding areas. There's also a good road and track oriented shop next to Olympic Park. I forget the name, but it's located immediately next to a gas station on the western entrance to Olympic Park. I don't know any roadies, and after 3 years as a bike courier in Atlanta and San Francisco, I'm fairly burned on the road thing for recreation.