The Alien Blaster is a unique, one-of-a-kind weapon that's been around since Fallout 3. You can only acquire this from an actual alien and its rounds are very rare that you should use them sparingly. Suprisingly, getting the blaster in the game is really easy.
Setting Oberland Station as your starting point, walk to the southeast. You have to look for the freshly ravaged trees, some of them will still be in flames.
From the crash site, if you follow the green blood trail, it will Lead to a Cave downhill. Go there and kill the alien to get his Alien Blaster and AB rounds.