Ranged weapons such as bows require ammunition for each attempted use.
The core rules make no mention of ammunition - it is intended that it be typically ignored. The core rules assume that a character wielding a bow, or even a firearm, has a sufficient stock of ammunition at any given time, and prefers to gloss over the details of tracking this information as it crosses the line into "too much paperwork".
However, if this level of realism is useful or desired in a campaign, then it is reasonable to request players track any ammunition they have. In that case, the following table defines the expected costs of purchasing ammunition from any vendor that would be expected to stock it.
Ammunition Cost
Stones (Sling) Free (readily available on the ground)
Arrows (Bow, Shortbow) 5 for 1 Crown
Bolts (Crossbow) 2 for 1 Crown
Charges (Blunderbuss, Flintlock) 1 for 5 Crowns (includes black powder, ball or pellets, packing material, etc.)