Fallout and Fallout 2
|Alien Blaster (Fallout 1 and 2 Version)|
|Ammo||Small Energy Cell|
In Fallout 2 the weapon can be looted off of an NPC in a random encounter.
It runs on Small Energy Cells, which is odd for an alien weapon.
 Fallout 3 & Fallout: New Vegas
|Ammo Type||Alien Power Cell|
The Alien Blaster within Fallout 3 is one the post powerful weapons in the game. Due to its 100% critical rate, almost all enemies can be dispatched in 1 shot. This is true, even when the weapon is in poor condition.
With the Broken Steel expansion, another one can be found inside Adams Air Force Base. It is within the armory, in a locked box (Hard difficulty) inside the Mobile Base Crawler. There are also Alien Power Cells inside the numerous ammunition boxes.
In Fallout: New Vegas, it can be found using the Wild Wasteland Perk north of the Horowitz Farm at a Mercenary Camp. Instead of mercenaries, The Courier encounters two aliens and one Alien Captain. The Captain has the Alien Blaster, which can be looted off him when he's dead. The aliens come complete with a crashed spacecraft.