Disruptor Stun Gun is a great weapon for stunning human enemies but you can also try to use it to shock greater alien forms and defense turrets. The weapon can be found in two places at the beginning of the game and is very well-hidden in both cases.
The first opportunity to obtain the disruptor appears right after you leave the apartment where the experiments on Yu took place. When you're in Simulation Labs of the Neuromod Division, you have to go to the Simulation Debriefing room shown in picture 1 (the first locked safe is located in the same place). The weapon you're looking for is hidden in a briefcase on top of the blue cabinet with recording devices shown in picture 2. Climb up and take the weapon out of the briefcase.
The second opportunity to obtain the disruptor appears in the later stage of exploring the Neuromod Division. Namely, it happens right after you reach the area where you find GLOO Cannon. After you've acquired the said weapon, move to the big room, turn right, and investigate the small roof hidden behind the "Transtar" sign shown in picture 1. Use GLOO Cannon to build a small tower. Climbing it will let you access the aforementioned roof (image 2).
Move to the end of the roof and start sending balls of glue towards the hanging lamp from picture 1. Ultimately, you have to land on it and then jump onto the neighboring upper balcony. Explore it. The disruptor can be found on the corpse shown in the picture 2.
Note - If you are unwilling to embark upon a quite complicated climb to the upper level of Neuromod Division, you can simply wait and proceed with the main story until you've received access to this part of the station. Check How to access the upper part of the Neuromod Division? in the FAQ section of the guide for more details.