The Fat Man is an iconic weapon and it has made a return in Fallout 76. Not all the Fat Man locations have been found as they can be random quest rewards. This mighty weapon doesn't always spawn at the same expected locations depending on the server you have joined.
Despite this, there is one location the Fat Man always seems to appear and it's near the Garrahan Mining Headquarters to the south of the Appalachia map. To get your hands on this Fat Man so you can launch mini nukes, assuming you have any, you need to balance your way along the suspended monorail until you reach a small platform. Sitting right there is a Fat Man waiting ready for you.
The video below shows exactly where you can pick this one up. This is at least a start prior to getting your hands on more from quests and other random locations as you traverse the Fallout 76 world.