Thanks to the good folks at Gameranx, we now know where to find the game's most effective Deathclaw-killing weapon, The Alien Blaster. Read on for the details.
This weapon can be found through a random encounter at Forest Grove Marsh, a flooded neighborhood filled with Ghouls south-southeast of Sanctuary Hills, about midway through the map. However, if you fail to follow the clues from that encounter, you can instead fast travel to your settlement of Oberland Station and travel a short path east to find a crashed alien ship. Follow the trail of splattered green blood to a nearby cave (the entrance will show up on your local map). The Alien Blaster Pistol is located on the floor. Be sure to kill the alien and loot its body for all the ammo.
There you have it. Just remember to save that ammo for the Deathclaws and Alpha Deathclaws. You're going to need it.