Look for thermal clips. They are practically everywhere, so you should be able to find some.
Thermal clips can also be refilled by finding power cells, or using a 'Weapons Loadout' locker on a mission.
^^ I thought the question was about cheats, was surprised to see those answers. My faith in humanity was shattered once more. Are there really people out there who didn't know about thermal clips? Anyway, if you want "classic" unlimited ammo, see this, method #3