Multiplayer games do not have any vendors scattered around, therefore players have to acquire ammunition from Ammo crates. Unlimited Ammo can be selected as a game setup option before starting.
Behind the scenes
It was revealed in an Insomniac Games PAX 2010 Q&A, ammo will not be the same from other games, or may even be removed completely. No ammo depletion was seen in the demo, however they have still to finalise on the ammo situation.
In the first Ratchet & Clank game, the ammunition you could pick up were unique to each weapon. However, from Ratchet & Clank: Going Commando onwards, all ammunition were represented by one cartridge.
In Tools of Destruction, Aphelion talked about Seeker ammo, that means theirs a posibility that fighting crafts must use different kinds of ammo.