Fibula pieces are obtained as an uncommon drop from Wallasalkis, found in the Waterbirth Island Dungeon. These are used to make skeletal bottoms by giving Peer the Seer 2 dagannoth hides, 7,500 coins, and 1 fibula piece.
Dropping monsters
Monster Combat level Quantity Rarity Wallasalki 98 1 Uncommon (1/64)