Out of the many stationery stores I've frequented in the past four years, I think my personal favorites are Artbox and Alpha. The stationery area at Yoongpoong Bookstore is also worthy of note for its selection. The aforementioned stores not only carry your basic office and school supplies, they offer candy, clothes, small electronics, picture frames, hair accessories, jewelry, and DIY art projects.
Even if you're not looking for something in particular at that moment, taking a stroll down the aisles might conjure up some future present ideas for friends and family.
Purchases from Alpha, one of my favorite stationery stores in Korea. And yes, that is a chick mouse-pad.