How can I reliably find a police car?

I confirm the information in @Timot_lambo answer, but I feel like a screenshot of the location on the map is required, because youtube videos come and go, but screenshots stay forever.
Credit goes to @Timot_lambo for the video he provided There is also certain fixer mission that leads you to a police station with 3 police cars parked. Unfortunately, in free roam the parking lot seems to have random cars. I'm not sure if you can start the mission, take the police car and quit mission while keeping the car, because once you finish the fixer mission, you can't re-do it (at least I don't know how?).

Recently I have had the same question as you, though after a bit of searching I found it, and there's even a secret car there! I have made a video showcasing the station/special vehicle:

In short, it simply shows that there is a police station, which has police cars parked outside, by a ctOS tower to the right-corner of The Wards, or just a bit right of the Rossi-Fermont hotspot. After researching more, I came across something else, once the ctOS tower is taken, the police station now spawns with a possibility of other cars, either the SUV & hidden car, or SUV & police cruiser, sometimes even with police interceptor. I have made another video showcasing this:

Thanks guys for the support, it's the first time I've been recognised :)