I first watched on Viki and the subs aren't great. Lots of errors, hard to follow and they CC the song lyrics at the same time which makes it even harder to follow. This is a dialogue-heavy show as it is and I'm really struggling to find the best subs. Hoping to introduce someone in my life to this gem and I want to make sure they're able to follow along easily. I love this drama so much I'd even buy the DVD if I had to to get the best subs. 

Kocowa has superior subs, but it's subscription only for older, non-airing dramas. They do offer a 14 day free trial if you're okay with watching the show with a time limit.

The best subs are the old Dramafever subs, which you can thankfully still download from Subscene. There are number of browser extensions - the best being Substital - that allow you to load .srt files and overlay them on almost any internet video stream.