Quarry Junction - Lying near the edge of a radioactive pool to the west of the quarry.
Nopah Cave - On the corpse of a super mutant master. Upon returning to the cave several days later, this super mutant may be found alive with another Fat Man and only a small amount of health. He continues to respawn every few days thereafter.
A Fat Man and 3 mini nukes can be pickpocketed or looted from a Boomer while they are sleeping in one of 3 tents near crops to the southeast of the mess hall & munitions storage. If killed, this Boomer respawns every 3 days, providing a steady source of mini nukes.
A Fat Man can be found on the body of a super mutant master near a cazador nest directly south of Jacobstown. This location is marked during the quest Bleed Me Dry. Facing away from the Jacobstown gate and climbing up and around the hills to the right will lead to the path. The area is also accessible by going west from Red Rock Canyon, over some very steep trails.
The Fat Man found in New Vegas isn't nearly as powerful as the Fat Man found in Fallout 3, doing 1000 less damage (unless you hit the target directly), and having an explosion radius of roughly half the size.
In Prima's Official Game Guide, this was considered a unique weapon.
The names of Fat Man and its mod Little Boy are references to the code names of the atomic bombs dropped by the United States on Japan in World War II. Little Boy was dropped on Hiroshima while Fat Man was dropped on Nagasaki.
Just like in Fallout 3, this weapon has been re-named as the Nuka Launcher in the Japanese translation of Fallout: New Vegas, due to its relation to Japanese history. However, the Little Boy mod still has the same name.
ps3 Sometimes when a second mini nuke is fired to a distant location, an explosion will not be seen. [verified]
xbox360 If you cripple your legs and go into third person, it will look like your character is holding the Fat Man, but there will be nothing there. Instead, it will be laying on the ground near you or floating in the air. If you fire it the nuke will go in whatever direction it is aiming. [verified]
ps3xbox360 Direct hit damage from the projectile does not get applied. This is a bug that applies to all non-hitscan projectiles that cause explosions. [verified]