Board Thread:Boruto Discussions/@comment-33589845-20171127205023/@comment-1458758-20171127222752

Well Danzo got his arm from Shin, and the eye in his head was the one he took from Shisui.

If I had to guess, given that all his clones seem to have Sharingan naturally, I'd say that he was probably implanted with one and then grew and harvested a load of them.