Some might argue that Hex Wrenches are the rarest item in The Division 2, simply because finding one is bloody difficult. That said, there is one location that I've found that often gaurentees one, and doesn't involve you having to log in and out constantly to loot a single filing cabinet. 

Simply head to Dark Zone East (Charlie Entrance), exit back into Judiciary Square, head immediately North and you'll see a ladder on your left side. Climb it, loot what's on the platform, then head into the office. Inside there's 5-7 filing cabinets that can be looted, which can all contain a Hex Wrench. If you don't get one the first time (unlikely) just log out, and repeat the process. Enjoy!