In Franklin Metro Utility, downstairs in the center of the second room, in a gated off area on a desk with a shotgun, some missiles, and a Stealth Boy.
In the locked weapons stash. The key can be obtained by completing the quest "Galaxy News Radio" if you already know James' location before returning to Three Dog.
In an ammunition container in the burned out bus near the red railroad car NW of the building that is north of the Office (not the same red car where Dogmeat is found). You can get the key from the random encounter "More Than Just Scrap", or you can just pick the lock (hard).
The skill book found in Regulator HQ requires the player to have the Lawbringer perk to be able to obtain it. This effectively reduces the total number of Small Guns skill books down to 24 for players that are not interested in choosing the Lawbringer perk.