You need to go to Lizzie's bar.
You can find it in Kabuki, It's a subdistrict of Watson.
The bar is in the central part of the district near the SUTTER ST fast travel Point.
The bar opens after 6 pm.

- First of all you can get it during the Quest ‘Automatic Love’ after you called Judy.
If you missed it you can still get it at any time after this Quest.

- First step go to Lizze's bar.

- Go to the back Room, go through the door and go down the stairs.

- Go through the first door in the basement.

- Go to the left, you can see the pistol lie on the table.
Grab it an have fun