Boss keys are always very close to the bosses lair in Phantom Hourglass because you have to carry it.
Isle of Gust (in the temple)
the island of gust is in the north western sea.
the triforce
You will find it inside the temple.
you collect the rare materials first and go see zuaz ISLE ABOVE THE ISLE OF GUST
You need to go to the nearby island and solve the puzzles there
You have to do the isle of gust and you`ll get it!
You beat the isle of gust.
You need to obtain the Iron Boots by completing Ice Isle, you'll also need the fire arrows.
Sorry there is no bow and arrow on the Isle of Gust because I just beat it a while ago theirs no bow and arrow
Zauz can be found in Zauz's island, north of the Isle of Gust
in the temple of wind on the isle of gust