The first copy of the weapon, alongside all three weapon mods, can be found next to a dead Bright Brotherhood member at
'Primm Pass'.
The second copy of the weapon, without mods, can be found on a shelf inside the
'Silver Rush'. The shelf is to left as soon as you enter.
The third and final copy of the weapon, again with no mods, is found inside a safe in the
'Lucky 38 Cocktail Lounge'. Note that this *replaces* the Plasma Defender which is originally found here.