You'll need to purchase the Saddlebag upgrade for your Bike so you can use it as a mobile ammo resupply point. The earliest one can be bought from Manny in Copeland's Camp, unlockable at Trust LV2. Once you have the upgrade installed, fill the saddlebag with ammo by purchasing it from any Merchant. This is an invaluable upgrade mid-game, especially when you have access to more reliable Weapons and when you're facing greater threats.
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