Firstly, I think you can find some brown or green bins with the words "의류 수거함". They are usually near houses or apartment buildings with other trash bins. Also, there are places like '아름다운 가게' where you can donate stuff. And if you want to ask somebody that sentence, 의류 수거함이 어디에 있나요?

의류 수거함은 어디에 있나요? or what about donate at 아름다운 가게 ? you can search on the internet where is close location 이름다운 가게 :)