In Paper Mario, you can buy it at Rowf's shop after Chapter 2 for 100 coins and it costs 1 BP to wear. When you wear it, you can use 2 items in one turn for 3 FP. In Paper Mario: The Thousand-Year Door, it costs 3 BP to wear and 4 FP to use but you can still use two items in one turn while wearing it. You can find it in Rogueport by using the boat panel behind the Trouble Center and at the Pit of 100 Trials on the 60th floor.