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
The alien blaster can only be obtained if you have the wild wasteland perk and it is located in the top left of the map and can easily be found by following the mountains east from brooks tumbleweed ranch and is carried by the alien captain and there are two other aliens accompanying him/it.
go to Google and type in Fallout 3 wiki. they'll have a detailed map. i can tell youu its north or northwest from greendale
the only alien blaster ammo is the ones u get wen u find the gun
or download mothership zeta
you can not find any energy cells in Fallout 3 after you find the ones with the alien blaster, you are limited to that number
alien blaster but ammo is hard to find after you have it all and its at the alien crash site
you go to the alien blaster location and jump on the alien space ship
The only ammo you get is where you get the Alien Blaster. But apparently there is more ammo near a truck where a Mr. Handy is. Hope that helps. :)
if its the alien blaster you want, its easy to find. watch this youtube video. http://www.youtube.com/watch?v=oTdu5ABpSEE
On the mother ship zeta expansion pack, if you don't have that you can't get them, besides one alien blaster with limited ammo at a UFO crash site
from the crashed alien ship next to the dead alien
You must have the WILD WASTELAND PERK and go to tumbleweed ranch and around where you would normally find the rare gauss rifle you will find aliens in which you must kill to receive the Alien Blaster
it repairs your weapons
Sadly you cannot get anymore ammo for the weapon anywhere else in tho mojave or surrounding areas :(
exactly North of Mine field.
north east of the map in an alien ship