South Korean posters are pretty hard to find.
I lived in Seoul from 1996-98 and there is no culture for collecting posters (maybe this has changed since then). It's very difficult to find posters and once they pop up they can be expensive.
Below are some examples of titles I was able to find (old pics, sorry for the quality). Since the printing quality of older posters is not very good and the artwork often rather crude, I did not continue searching for them but focussed on Japanese posters instead.
Armin