There are 2 ways to get the Khvostov 7G Schematics. The first is to dismantle the Khvostov 7G-02, the first gun you receive when you create a new character. Dismantle it and you will receive the schematics automatically.
The other way is to hunt down and kill Broodmother Wizards in the Plaguelands. Killing them has a chance to drop these as well. They appear in numerous areas and have a short respawn time. Most are trying to get to a particular location. If they make it, they will simply vanish and you will need to wait for the next one to spawn.