The warrior helm can be bought at Skulgrimen's Battle Gear shop in Rellekka for 78,000 coins. It can also be obtained from another player or the Grand Exchange (the latter is recommended, as this helm is 32,775 less than buying it from the store).
With the second Summoning update a player can have Pikkupstix enchant the helm at which point it becomes a warrior helm (e) and can hold up to 700 scrolls. When charged it takes on a lighter colour and becomes a warrior helm (charged). It isn't a very common helmet worn by players, as some prefer to wear a dragon helm or berserker helm instead.