I don't know about new sheets, but several of the older Chimneyville sheets contained markings for some agencies that were consistent over the years from the 50's through the 70's. Some of those would work.
And another thing to remember about vintage police and fire vehicles: In the 30's to 40's and somewhat into the late 50's, many county and municipal agencies ran rather simplistic markings . . . because they were cheap. The markings ranged from hand-lettered jobs done by a local sign painter to mail-order badges of a generic nature obtained from suppliers who printed up only a couple of different logos that could be made unique by adding the town or county's name.
Especially in the 30's through the mid-50's, many departments may have only had the words "Police" or "Fire Department" or "Sheriff" emblazoned on their cars. They figured if you were in the town of Podunk and saw a "Police" car, you should darn well know it's the Podunk Police Department's car. No sense in spending a bunch of money (sign painters charged by the letter) for frills.
The unique and sometimes wild markings to which we've become accustomed only began evolving in the very late 60's or early 70's, and have only become commonplace in the last thirty years or so. It was only the really big or really rich agencies that were into custom graphics before then.
In short, there's nothing wrong with building a plain, black or white, or black & white, or just about any other color 57 Ford with the word "Police" on the door and calling it done. It would be as accurate as anything. It's your model, your town, build it as you see fit.