Install Fallout: New Vegas on PC
Install the DLC “Lonesome Road”
Start or load a save where the Courier can access the Lonesome Road content
Progress through Lonesome Road until reaching the Divide
Find the Pimp-Boy 3 Billion in the marked location during the DLC questline
Pick it up from the container or corpse where it is found
Equip it from the Pip-Boy inventory
If you want it without replacing your current Pip-Boy, use a mod or console command on PC