Wooden Arrow

The Merchant stocks them the moment he moves in, which is the answer most of the time, and they turn up loose in pots and chests all through your first caves. The Skeleton Merchant carries them too, but only on some moon phases, so he is a bonus rather than a plan. Pincushion Zombies drop them as well if you are out on an ordinary night.

Any bow that rewrites your ammunition makes expensive arrows pointless. The Molten Fury converts whatever you load into flaming shots, the Marrow and the Ice Bow each impose their own projectile, and the Pulse Bow overrides every arrow type in the game. Loading a Holy Arrow into one of those gets you the same shot you would have got from wood, at many times the price. On those weapons wood is correct and stays correct forever.

Every better arrow is built on this one, so keep a stack running long after you have stopped firing them raw. In Hardmode the supply problem disappears entirely: bundle them into the Endless Quiver and the ammo slot never empties again. They can also be dropped into Shimmer, which returns them as Shimmer Arrows.

The projectile a Dart Trap spits at you is drawn as one of these. In the pre release build of the game they could not be crafted at all, only bought or scavenged out of pots.