One place to get iron arrows quickly is in the Stronghold of Security, located below Gunnarsgrunn. In the first floor of the Stronghold, players can find Minotaurs, which can drop from five to twenty iron arrows when killed. Players can also randomly find them while wearing one of Ava's devices.
Similarly, Lumbridge offers the same service, with iron ore available in the Al Kharid mining site, feathers obtainable in a farm north of the city, trees in-between these two areas, and a furnace and anvil for smelting and smithing the ores.