You can get an Alien Blaster on the way to Old Olney (northern, 1/4 from left edge of map). It's Southwest of Old olney, directly north of MDPL-13 power station, and southwest of the cliffside shacks and Oasis. Birds fly over it as well, like with other major landmarks.
It's alongside one of the ruined roads, by a ruined house. There will be radiation when you get near it, and before that, you will get the Recon Craft Theta Beacon signal on your PipBoy radio too, when you're close enough.
The other alien blaster you can get, Firelance, is a random event. People have had it happen at the Anchorage War Memorial, but I had it by Minefield. When that happens, if you're looking up you'll see an explosion in the sky, sounds like a rocket. What will follow is some RAD in your area +1, and then you need to scan the vicinity for the Firelance (a brown gun), and the alien power cells, since the Firelance itself has no ammo.
If you have Dogmeat, he can help you find it. Also, I think there's a higher likelihood that you will experience the random event, the higher your luck is. I have 4-5 characters in Fallout 3 and so far (and one of mine is level 27 and has NEVER experienced it) only one of my characters, a level 8 has had the Firelance alien crash encounter.