A vial of water is an item made by using a vial on a water source. It is mainly used for training Herblore.
Vials of water can be purchased from the general store in Ardougne, Shilo Village, Rellekka, Jiminua's Jungle Store, and from Jossik in the Lighthouse. It can also be purchased from the Nightmare Zone using 145 reward points. You may now buy them in packs of 100, called a water-filled vial pack, which starts at 261 coins.