Where is this gun?

you have to have wild wasteland,and then I found 3 aliens on the mountain behind,i think,vegas square,the alien captain has it

what about the kid in freeside chasing the girl???

Euclid's C-Finder
More effective than the Alien Blaster, but more more complicated to obtain.

How do you obtain ammo for it if you're not using Wild Wasteland? I bought it off him, and one of my companions has been carrying it for what seems like ever. I remember it needing some kind of ammo that I had never seen before, and I had none.

The location of it is to the north of the Horowitz Farmstead. If you don't know where that is, it is in the upper left portion of the map.

Exactly how to get it
1.Fast travel or get to Horowitz Farmstead
2.From there go exactly to the North
3.As soon as you have gone as far North as possible start to head East
4.You will soon come across 3 aliens. Kill them all, then search the Captains body.

Note: You need the Wild Wastelander perk in order to obtain this item.
Note 2: The Captain has ammo on him. Not very much, but at least it's ammo.