Mewtwo is found in the Pokemon Village. Cross a small stream on the western side using the Surf HM to reach a cave entrance. Mewtwo will be right at the entrance of the cave. Note: A man will block the cave entrance until you defeat the Elite Four. Mewtwo is Level 70 so use a Master Ball to easily capture him, otherwise it will be a long battle and require plenty of Ultra Balls. Onces you capture him, you will also get the special stone needed to activate Mewtwo's Mega Evolution.