Power Punch is an item that is found only in Paper Mario: The Thousand-Year Door.
It can be dropped from various enemies or purchased at the Souvenir Shop in Glitzville for 15 coins. Also, it can be made when Zess T. mixes a Mini Mr. Mini and Point Swap. Sometimes Charlieton is carrying one, but it is quite expensive at times. The Power Punch makes Mario or a Partner Huge and increases his or her Attack power by 2. It wears off after two turns of use.