Costco in Hanam, Seoul
Hanam Costco is as extensive as one in the US and seems to carry as many products. The first Costco that I went to was one in Yangpyeongu, but it was small, and it did not have a food section. Hanam Costco is far away from the center of Seoul. It will take at least about an hour trip by bus or subway or a combination of both. Nevertheless, I was able to buy the same vitamin and fish oil that I got in the US, though it was more expensive.
Hanam Costco: 344-1 Pungsan-dong, Hanam-si, Gyeonggi-do, South Korea