Finding a great tailor is not an easy thing. Especially when you are a foreigner in Seoul. When looking on forums, you will get many people telling you to check the tailors in Itaewon (the foreign district of Seoul). Don’t go there. The quality is bad.
A friend of mine, who used to be a fashion merchandiser for Lotte Department Store recommended me an awesome tailor named Knockers. The good this is that the owner does speak English. He is also a fashion advisor for one of the Korean TV show. The whole experience of getting a custom shirt (I never tried a custom suit from him) is so pleasing. I recommend you to check this tailor.
South Korea, Seoul, Jongno-gu, Gye-dong, 계동길 ２３