I've found the contact information for two doulas in Seoul. From what I've heard, you can't use doulas at the military hospital on Yongsan Garrison. I don't have confirmation of that, so if you are going to use the 121 and want to use a doula, make sure to ask the question. The two numbers:
Keun Hae (she also teaches Lamaze and breastfeeding) 010-5416-5155. She speaks both English and Korean. She is not taking on many client right now because she had a baby last year (plus has three other kids), but she is a good resource for all things pregnancy and baby in Korea.