He Got it From Obito when he was on a mission when he was younger. Obito got crushed by a rock and gave kakashi his Sharingan as a gift before he died. Obito had the Sharingan because he was an Uchiha.
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Danzo ordered Itachi to kill his clan or the destruction of the whole ninja world could occur. Itachi did without hesitation and then apparantly danzo took the sharingans of the many dead uchiha and put them into his arm and one in his right eye.
It say it in the manga he got it from Uchiha Shisui who is the guy Itachi is supected to have killed to get his Mangekyou Sharingan.
He got all those sharingans fron uchiha members killed during the Uchiha massacre.
Danzo only has 1 sharingan eye on his right side also he has some kind of special arm (his right arm also) his sharingan first shows on naruto shippuden episode 199 but it only shows for a few sec when he kills these assassins with his eye.
Episode 208
This fight between Danzo and Sasuke happens in Naruto Shippuden Episode 209: Danzo's Right Arm. More information about Itachi comes out in this episode as well.
it is not yet known but there are theory's. One is that he dug up Obito and got the sharingan from him. no one from Konoha would do this so he could have gotten the transplant from orochimaru. but i think that is unlikely because orochimaru wanted the sharingan himself. This may not be true because Obitos body was completely crushed. Another theory is that orochimaru made a sharingan and gave it to danzo for testing purposes. But this is too far-fetched to believe. Another is that Danzo is Obito which would would explain why one of his sides is wrapped and he got a normal eye from somewhere else (Obito gave one of his eyes to kakashi.) but Obito would be too young because danzo held a grudge against the 3rd hokage. My theory is that Danzo has a connection with "tobi" or madara. tobi only shows one of his eyes through his mask so it is possible that madara gave his other eye to danzo in an agreement. I don't think that this is the case though. We will just have to wait and see.
Danzo does not have a Sharingan, Byakugan, or special weapon. However, he controls the ANBU Black Ops and has a wide range of informants. The anime suggests he was once very powerful but has injuries to his right arm and eye. He is seeking Kabuto or Anko, who would have acess to Orochimaru's powerful jutsu and medicines. Anko also opposes him, so she is a loose end.