The Bullet Charger can be found in Reboldeaux, The Port of Coimbra and The City of Auch. When you click on it, a dialogue box will open allowing you to refresh your bullet supply without going through all the commands associated with the merchants.
Simply select the kind of bullet you require, how many rounds you want to have in your inventory, and the Bullet Charger will make up the difference.
Note that this works differently to buying bullets. When you select 10,000 rounds for example, if you already have 120 rounds in your inventory, you will only purchase 9880 additional rounds. Make sure you have enough vis in your inventory to buy the bullets!