REQUIRES FALLOUT NEW VEGAS!!!
If you do not own, I suggest getting it from GOG.com but Steam should work just fine. You must extract the bsa files named Fallout - Meshes, Fallout - Textures and Fallout - Sounds and GunRunnersArsenal - Main, all of which can be found in the Data folder for New Vegas. I recommend using B.A.E. https://www.nexusmods.com/fallout4/mods/78/

Required Files
meshes\weapons\1handpistol
Plasmadefender.nif
meshes\nvdlc05\weapons\1handpistol
plasmadefender1.nif

sound\fx\wpn\plasmadefender\fire_2d
wpn_plasmadefender_fire_2d_01
sound\fx\wpn\plasmadefender\fire_3d
wpn_plasmadefender_fire_3d_01

textures\weapons\1handpistol
1stpersonplasmadefender_d.dds
1stpersonplasmadefender_n.dds
plasmadefender_d.dds
plasmadefender_g.dds
plasmadefender_n.dds
textures\interface\icons\pipboyimages\weapons
weapons_plasma_defender.dds

Place the files in the files in the following locations in your Fallout 3 Data Folder and replace the blank placeholder file.

meshes\weapons\1handpistol
Plasmadefender.nif
meshes\nvdlc05\weapons\1handpistol
plasmadefender1.nif

sound\fx\wpn\plasmadefender\fire_2d
wpn_plasmadefender_fire_2d_01
sound\fx\wpn\plasmadefender\fire_3d
wpn_plasmadefender_fire_3d_01

textures\weapons\1handpistol
1stpersonplasmadefender_d.dds
1stpersonplasmadefender_n.dds
plasmadefender_d.dds
plasmadefender_g.dds
plasmadefender_n.dds
textures\interface\icons\pipboyimages\weapons
weapons_plasma_defender.dds

The Plasma Defender has a chance to be found on Enclave Officer's. There is also one found in the Operation Anchorage Armory.
There is a Unique Varient called the Glock 87.
What Makes it Unique
Has double the clip size, an extra 50 to Durability, slight increase to damage, slight increase to base VATS To-Hit, and increase Crit Chance. 
The Location is in the spoiler below

Spoiler:  

Show

Can Be found in a Raider Camp south of the Flooded Metro.

Compatability
Outcast Collection Agent Expanded - I added the Plasma Defender to the list of technology you can turn into the Outcast. If you want this, please downloadthe file main file that specifies.