They can be obtained either by buying from the Ranging Guild or the Void Knight Outpost, trading with another player or as a monster drop.
The Steel Javelin (鋼の投撃槍 Hagane no togeki yari lit. Steel Throwing Lance) is an enemy-exclusive ranged Lance from Fire Emblem Fates.
They can be fletched by attaching steel javelin heads to javelin shafts at level 32 Fletching, granting 75 experience per 15 javelins made. They can also be obtained by buying them from the Ranging Guild, the Void Knight Archery Store or from Oobapohk's Javelin Store after Monkey Madness II.