Ribcage pieces are obtained as an uncommon drop from Wallasalkis, found in the Waterbirth Island Dungeon. These are used to make skeletal tops by giving Peer the Seer 3 dagannoth hides, 10,000 coins, and 1 ribcage piece.
Dropping monsters
Monster Combat level Quantity Rarity Wallasalki 98 1 Uncommon (1/64)