Since the Fat Man is a simple variation of its weapon type and is less powerful than the Nuka-Nuke Launcher or Big Boy, it is found relatively frequently in several locations.

Mini Nuke Launcher Would you like to further irradiate the wasteland? Look no further than Big Boy, your personal mini nuke launcher. Big Boy is a variant of the common Fat Man heavy weapon ...

There's a new mod for Fallout: New Vegas which can only end in utter chaos as it provides all NPCs with their own mini nuke launcher.