Innis Free (everywhere), Tony Moly (best packaging), Skinfood, Too Cool for School and Etude House were my favorites for quality/affordability balance. Literally hundreds of cosmetic stores line the tourist and shopping areas in Seoul. Save some space at the airport for duty-free items. Most stores will give you at least one free sample--to score more, go directly to the stores and go in the mornings before it gets crowded. If you spend a lot in one place, you may get full-sized items as samples or even cute tote bags like the one I got below after spending a bunch at Tony Moly!