Receive one from a barbarian spirit after lighting a pyre ship with Chewed bones. Though this method has a much greater chance of obtaining a helmet compared to slaying Mithril dragons, Chewed bones themselves can only be obtained from Mithril dragons as a rare drop.
The dragon full helm used to be the most expensive piece of dragon armour, but was recently overtaken by the Dragon platebody. Many players thought the helm would fall in price a while after its release, but the dragon full helm, being a very rare drop from mithril dragons, showed no significant signs of a price drop until the release of PvP worlds. Then, as a result of many dragon full helms coming into the game from PvP worlds, the price dropped significantly, from over 50m in September 2008 to under 10m by May 2009. Then, on the 16th of June, 2009, dragon full helms were removed from the PvP world drop tables.
Shortly after the release of PvP worlds, many players believed that dragon full helms were a drop from other players. Initially, a Jagex moderator denied this, however another Jagex moderator later confirmed that dragon full helms were indeed dropped on PvP worlds. On 16 June, 2009, the full helm was removed from the PvP drop table.
When the dragon full helm first came out, its stats beat the Third-age full helmet's original stats in each section. Jagex then updated the 3rd age full helmet, which is now slightly stronger than the dragon full helm.