Have you been on the forums at homebrewkorea.com? I used to use them when I was in Korea. I was reading some Korean homebrewer post about trying to find CO2 and him trying the welding shops but getting non-food grade CO2. Not sure if that matters, to you or to anyone, but he said he eventually called a beer bar in Hongdae called Craftworks (I think) and asked them where they got their CO2 tanks. The place he fills in in Hapjeong, on line 2, near Hongdae.
His number is 02-334-3337. He doesn't speak English. You can try: "co2 실린더 다시 채우수있어요? " ---> co2 see-lin-duh-leul dah-she- chay-oo-soo-ee-suh-yo?
Then wait for a yes or no. Nay, an-yo respectively.
서울 마포구 망원동 57-419새주소
They have a branch in Bucheon as well. Not sure where in Korea you are stationed. But there are probably branches nearby any base. The number above will have more information. Also try Joongang Keg in the Joongang Market area. 중앙시장. – 중앙케그 (Joongang Keg), (02)-2238-8339/011-216-9536. They sell kegs and Co2 tanks. Can probably help you too. The market is the industrial mess of a market area just north of Sindang Station (line 2, central Seoul). It's about a 2 minute walk from exit 2. Looks like the place is called 중앙산업가스
If you're in Daejeon--https://www.facebook.com/groups/112599575486895/permalink/308205619259622/
Hoang - Local gas company in Daejeon for CO2 refills. They're friendly folks and will refill a 5lb. tank for about 5,000won. Google Map
Kimberly - Do they sell tanks, too?
Hoang - they don't sell tanks, but you can order a co2 tank & regulator set from these guys: http://www.fda.co.kr/new_daesan/new_daesan.htm
대산 코포레이션 - 콜라기계, 주스기계, 슬러시기, 제빙기, 커피머신, 정수기, 필터, 산소수기, 탄산수기, 맥주기계, 행사용 기계 렌탈, 대여 www.fda.co.kr
Hoang - The owners name is 이완응. Their bank account is KEB 278-18-099806
Kimberly - Look like they sell kegs, too