This shotgun can be located in the town of Quincy's detention center, inside one of the prison cells in a chest.
It can also be found in random encounters while traveling the Wasteland directly around Bunker Alpha once the Brahmin Wood mission has been completed. Ghost-like "citizen-thin" NPCs often carry one each (along with a stimpak and some 12 gauge shotgun shells). If you can't see it in the initial steal inventory window, let one of your team members get caught stealing from that NPC; on the next turn when you enter the Steal window, it will be there. You need about 80% Steal skill to be able to grab the weapon. If the NPCs wander around a lot, enter turn-based mode by pressing Enter.
In real life, the Neostead is NOT an automatic shotgun, it's a pump-action shotgun with a "forward-back" handguard movement instead of the classic "back-forward". Its main distinguishing feature is a unique double magazine, allowing the shooter to immediately switch between two different types of ammunition (ie non-lethal shells for riot control, with lethal rounds held in reserve for emergencies). It was developed by two South African engineers, Tony Neophytou and Wilmore Stead, in the early 1990s.