Are you talking about single-player or multi-player? For single-player, there are usually ammo crates scattered around any places where a combat is going to take place. In multiplayer, the maps all have several ammo crates scattered around the arena. They're usually tucked away in hallways or the outer rooms. Once you play the maps a few times, you get a feel for where they are. When in doubt, follow your teammates between rounds. Chances are they are all running towards a box.
Keep in mind, the boxes have a limited supply so you may not all be able to reload from the same box at once. However they do refresh between rounds.