From the Super Mario Wiki
The Bracer is an item from Super Mario RPG: Legend of the Seven Stars. It can be purchased for two Frog Coins at the Frog Coin Emporium at Tadpole Pond. Using it increases a character's defense by 100%, and it can be used only during battles. It can also be acquired by winning it from Bluebird, Magmus, and Spinthra.