Use a room in city hall, or any of the police departments. They are empty almost all of the time.
For a private enforcer group, you may want a seperate business front, for which you need only a small property.
You seem to be pursuing a very "I am a law enforcement group and hence I get free stuff" approach, which so far has only worked with the FBI. Look at SASD, SACS and others, they got very little script support if any.
Work for a long time to make it good. And I mean, work at it for years (not a figure of speech). If you can prove you have the ability to run a good group, then good thing will eventually come to you.
The fact you're a law enforcement group means you are free to use the state buildings. Keep in mind that criminals are obligated to purpose their own headquarters to have any official front at all, whereas you can basically pick a nice looking desk in the LSPD (not Vince's D:) and say its your operations area.
Good luck!