Where is alien blaster fallout 3?

The normal alien blaster is at the site of the alien space craft. There is a grid of power lines west of minefield. follow them north until you get the alien radio signal, i forget what it is called. Then go off to the right a little bit and you'll find the space craft and the alien blaster, along with about 110 ammo for it.

The fire lance alien blaster will be found at a random location selected by the game. I was lucky enough to get mine at the super duper mart. The radio signal is called Recon Craft Thefta

Or, North of Minefield