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Its in Lord Jabu Jabu's Belly....a dungeon. After you complete the Great Deku Tree Dungeon, and Dodongos Cavern dungeon, then you go to Lord Jabu Jabus Belly in Zoras Fountain. In the Dungeon, after you find Princess Ruto you to through and get the Boomerang.

But to get it without finishing the dungeon, go through until you meet Princess Ruto, let her fall and just go into the back of the room your in and you should come to a hall with a bunch of break off halls....Go I think left and defeat the stuff in each room until you get the boomerang...