Archer helms can be bought from the Rellekka Helmet shop for 78,000 coins after completing the Fremennik Trials quest. Level 45 Defence and completion of the Fremennik Trials are required to wear it.
With the second Summoning update, a player can have Pikkupstix enchant the helm; at which point it becomes an archer helm (e) and can hold up to 70 scrolls. When charged, it takes on a lighter colour and becomes an archer helm (charged).