The Alien Blaster can be obtained next to the body of a dead alien lying a few feet away from his crashed spaceship. The Alien Crash Site is found due north of MDPL-13 Power Station and northwest of the Greener Pastures Disposal Site. There are only 120 Alien Power Cells found at the crash site. In the Broken Steel DLC, another one can be found at Adams Air Force Base. It is held in the armory inside a locked box (Hard lockpick) in the giant Mobile Base Crawler. There are also alien power cells in the numerous ammunition boxes in the room. There are 4 ammunition boxes that contain 9 power cells each (significantly more if you have the Scrounger perk), and the Armory Master also has some on his corpse. There is a fairly good chance the Armory Master, a Security Officer or a member of the Enclave Squad Sigma will pick up the Alien Blaster before you can make your way to the armory where it is held. If the gun container is empty, be sure to check the corpses of everyone in the vicinity.