Mithril darts are Ranged throwing weapons made from mithril that are only available to members. Mithril darts require level 20 Ranged to wield. They can be obtained by either using the Smithing and Fletching skills or trading with another player. Mithril darts can be poisoned and can be wielded with a shield or prayer book for protection.
Mithril darts are one of the most cost effective Ranged training weapons for lower level players, as they have an extremely quick rate of fire and cost very little compared to similar training methods like throwing knives or arrows. They can be bought in large numbers from the Grand Exchange, costing less than half of what similar tier ranged ammunition costs.
To create mithril darts the player must have completed The Tourist Trap quest and have at least level 54 Smithing and level 52 Fletching. Players can smith mithril bars into mithril dart tips. Each mithril bar creates 10 mithril dart tips and gives the player 50 Smithing experience. Feathers then must be added to create the mithril darts. 11.2 Fletching experience is rewarded per mithril dart.
For an exhaustive list of all known sources for this item, see here