Ammo is easiest to buy from the castle location on the map. The icon looks a bit close to the icon for a quiver of arrows, but a quick hover over a possible candidate should clarify. I’m not entirely sure when ammo is deducted from the bin to refill quivers though, and since i have not paid enough attention to say accurately, I won’t speculate. Just know that ammo is easiest to buy at the castle, and sometimes the city. It’s hard to find it anywhere else.
