Shakespeare relied on a book about Richard the Third by Thomas More. It was More who helped destroy Richard’s reputation. Some say inadvertantly (his book might have been a satire) or deliberately to help solidify the kingdom of the Tudors. It was never published but found upon his death.
I’ve heard that Richard’s body has already been identified at Leicester University. There is also a rumor that he was buried with a piece of jewelry that identifies him as king.