Sold by Weaponsmith Ornifex for 60 souls each.
Deals 160 points of Fire damage, plus 50 physical damage.
If it can be avoided, don't use these arrows on a purely physically damaging bow, as splitting damage types can severely harm your damage; Iron Arrows would be a much better choice. A bow that deals fire damage is an excellent fit such as a bow that is fire Infused.
Fire Arrows can also be used to blow up barrels which can deal large damage to enemies and destroy walls to open up shortcuts such as the one near Steady Hand McDuff's workshop. They can also be used to ignite oil patches from a distance, which can be useful in The Gutter and Black Gulch.