Where did Madara got his left eye after giving both of his real eyes to Nagato? Madara said that he gave his real eyes to Nagato during his meeting with Obito, and that his current left eye was implanted recently.

Where did he get that eye?
Madara had a huge stockpile of Sharingan, presumably collected from the bodies of various Uchihas over the many years he has been alive. 

I would expect he used one of these, and that the reason he collected so many is so that he can use Izanagi and other techniques that cost the user an eye quite freely.