There is a guy in the Giants forest. In that one area with the giant root/branch that leads up the wall if you look in the far right of that area there is a hole in the wall. You have to jump off the root, to the roof on the right, then jump from that to another roof. You should then be able to go in the cave. Watch out for a giant boulder and he's at the end of the path. Just keep talking to him til he gives you a key.