There are many Buddhist temples with the usage of 불교의 만자, but I can't find any informaiton about where the oldest symbol is. The book by Thomas Wilson addresses the history (pre-history) but does not specifically address Korean history of this.
Do you want temples or literature?
According to this page, there are many swastikas in Goryeo Dynasty Buddhist paintings.
And this painting is from the 14th century. https://ko.wikipedia.org/wiki/%EC%95%84%EB%AF%B8%ED%83%80%EC%82%BC%EC%A1%B4%EB%8F%84_(%EA%B5%AD%EB%B3%B4_%EC%A0%9C218%ED%98%B8)