I would direct you to the end of the Gospel of John, the Acts of the Apostles and the Letter of Peter. Then again, maybe those aren't the "proper sources" for your not hearing much about him.
I said that we dont hear MUCH about Peter after he betrayed Jesus and we dont. I didnt say that we never hear about him again. He betrayed Jesus and was forgiven, however, his position within the fold diminished considerably after that.
This still does not answer my original question were is there physical proof that Peter ever visited Rome?