I think that the friend theory could be right.

But my own theory is that he got that eye from Orochimaru and his gang.
Why didn't jackson take them for himself. Well because like on Kakashi the Sharingan is always on, so Jackson would be always using the sharingan and he probably wished for two working Sharingans. Remember that Oro has promised power for many people, well... atleast for a few. And Kakashi probably would be interested in Sharingan because he wanted to become even stronger in order to stay ahead of Gai. the scar he just got from a battle. Only loophole in that theory is that why did Orochimaru let Kakashi get away with the Sharingan without killing him in the first moment of rebellion? Well perhaps Kakashi did something in favor to Orochimaru in exchange for the Sharingan

Those are my theories. Hope they are near the truth.