The Alien Power Cells are hard to come by, but you can find some from these locations:
- 120 (10 stacks of 12 cells) found on the Alien Crash Site (You probably picked these up already.)
- 96 (8 stacks of 12 cells) east of Fort Bannister. They are in a radioactive crater, under an overturned truck. Look for bombed out buildings, when you find them, adjust your direction north. Some Enclave Scientists can appear here.
- 2-22 (without Scrounger perk) or 2-122 (92 on average), in two ammo boxes in Fort Independence, in the Outcast weapon room, where you find the Miss Launcher.
A full list of locations can be found here: http://fallout.wikia.com/wiki/Alien_Power_Cell