According to tradition, John Smith received the Book of Mormon from the Angel Moroni, himself.
I choose to believe that it is tradition, passed down through the teachings of the Apostles, and later their disciples. You may not believe it, but I don't know of any evidence that it isn't true.
Just like there's no evidence that the lost Plates of Nephi and the Book of Mormon aren't true either.
It's been less than 200 years. Produce them for me.