By doing the Call to Arms quest, it is possible to potentially get an infinite amount of fusion cells. At the end of the quest, one must press a button to stop the waves of synths attacking Paladin Danse. By simply not pressing the button, unlimited waves of synths will appear, containing fusion cells with each one. The synths will despawn if left for too long.
A miniaturized fusion reactor is similar in appearance to the microfusion cells from previous Fallout games.