On windfall island, first talk to a guy with a blue coat, he has a little shop in town, its outside. he will tell u to free his friend, then in the front of the island there is a hill with a grave, near that is door into a dungeon

once you free tingle go back to the vendor and he will offer to sell you "it". it is te sail, he will offer it to you for 80 rupees after haggling. hope this helps!

The sail is sold by the guy dressed in cold weather clothes. He is close to the cafe.