The Zora Helmet is part of the Zora Armor set. The helmet cannot be purchased like other armor pieces. Instead, you will need to locate a chest that contains this helmet.
The chest that contains Zora’s Helmet can be found at the bottom of Toto lake, which is just above the waterfalls, north of the Zora domain. You will find the chest toward the center of the lake near a broken-down wall. You can use the magnetism rune to pull the chest out of the water in order to get the helmet inside.
The following video will show you exactly where to find this helmet.