The Master Sword in Twilight Princess is located deep within the Faron Woods in the Sacred Grove, which are the ruins of the Temple of Time
This shield becomes available once Link reaches the point where he can begin
the Goron Mines. If you begin the dungeon without it, you may get a prompt
from Midna about needing protection. It can be purchased for 200 rupees from
the Kakariko General Store once Malo takes it over.
here is a guide if you need a walk through.