I'm wondering where to find the alien weapon in fallout 3. A friend of mine played a new game and put it on very difficult and then he went to Super Duper Mart. He found some raiders, killed them and took a "Flaming spear" gun with alien energy as ammo. I cannot find it in my game. Is it because I play on normal difficulty?
You might be talking about the Firelance. You can read up on it and how to get it in the link. Basically it is a part of a random drop encounter.
It could also be the Alien Blaster, which is very similar to the Firelance but easier to acquire. You have to get it from a crashed spaceship (which is very radioactive.)
The Alien Blaster can be obtained next to the body of a dead alien lying a few feet away from his crashed spaceship. The Alien Crash Site is found due north of MDPL-13 Power Station and northwest of the Greener Pastures Disposal Site. There are only 120 Alien Power Cells found at the crash site. A few cells may be obscured by the ground (standing vertically and partially underground).