If anywhere is going to sell them it would be all the flower and plant shops near the farmers market in Ban Song. Which is at the top of the Suyeong river. Closest place of note to it would be Dong Nae station, but you would probably need to get a bus from there.
According to Naver maps the 115-1, 189-1, 129,189, 183, 73, 129-1 all go there. Where they go from I have no idea.