There seems to be very little information available in English about double bass shops in Korea. Here's some information about luthier shops and factories located in Korea. Most of the stores are concentrated near the Seoul Arts Center (예술의전당). One can also find basses in Nakwon (낙원상가), which is a huge building that sells all kinds of musical instruments. I personally haven't had good experiences there.
This is the one I go to for setup and repairs. My current bass is made by them. It seems to have forked into 2 shops lately.
a1. Bas String (now France String). (바쓰악기 now 프랑스스트링).
http://www.basstring.com/
http://smartstore.naver.com/francestring
a2. Bass String. (베이스 스트링).
https://map.naver.com/local/siteview.nhn?code=13009664
b. Hill String. (힐 스트링).
http://hillstring.modoo.at
https://www.instagram.com/hill_string/
c. Son Choi String. (손앤최 스트링).
https://cafe.naver.com/gubbioviolin
d. Wood Strings. (우드악기).
https://smartstore.naver.com/woodstrings
https://blog.naver.com/holylovess
e. Bajo Bass Club. (바조베이스).
http://www.bajobass.com/
f. Linear Kontrabass Luthier Workshop. (라이너베이스).
www.linear.co.kr/
g. Atelier Hagen. (하겐현악기).
http://en.hagen.co.kr/
h. Jeong Hyun. (정현악기).
I used to own a factory bass from here. I used it for many years and it never let me down.
http://www.jeonghyunmusic.co.kr/index.php
i. Shimro. (심로악기).
http://www.shimro.co.kr/shop/list.php?ca_id=1040
That's all for now.