Prey's Disruptor Stun Gun is great for taking down mind-controlled humans in a non-lethal way, and you can find it pretty early on on the second-floor of the Nueromod Division simply using your GLOO Cannon as stairs.
But you can find it even earlier than that.
The first time you can obtain one is in the Simulation Debriefing Room, which is accessible right after you escape your apartment. This is the room with the Safe and the whiteboard that has the erased code.
On the same wall as the whiteboard, you'll see a large machine. Jump up to the top of this machine, and you will find a note from Alex, and a briefcase containing, in addition to other good items, the Disruptor Stun Gun with a ton of ammo.
Now get stunning.