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
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
it repairs your weapons
alien blaster but ammo is hard to find after you have it all and its at the alien crash site
Fallout 3 was created by the same people, Bethesda Studios. Many call it "Oblivion with guns."
minigun, fatman, experimental mirv, missile launcher,
you go to the alien blaster location and jump on the alien space ship
exactly North of Mine field.
north east of the map in an alien ship
There are about 50 at Fort Independence.
I recommend downloading the last downloadable content then get everything on Mother Ship Zeta there are really powerful weapons if you look for weapons with peculiar names such as the alien atomizer and the Disinigrator.
if its the alien blaster you want, its easy to find. watch this youtube video. http://www.youtube.com/watch?v=oTdu5ABpSEE
The best locations would probably be The Strip, because you can have lots of fun, and spend a lot, and get a lot. :)