Skull pieces are obtained as an uncommon drop from Wallasalkis, found in the Waterbirth Island Dungeon. These are used to make skeletal helms by giving Peer the Seer 1 dagannoth hide, 5,000 coins, and 1 skull piece.
Dropping monsters
Monster Combat level Quantity Rarity Wallasalki 98 1 Uncommon (1/64)