From GameWatcher: As Fallout 76 shows, a post-apocalyptic scenario doesn’t necessarily come with a lack of weapons. While ammo for some is easy to come by, others require ammo like the plasma core which is harder to find.
Here’s what we know about how to get yourself one plasma core or more in Fallout 76.”
Source: N4G PC Fallout 76 Plasma Core – How to Get Plasma Core