Most police, all major cities and most minor ones too, have moved to a trunked digital system. In these systems the sound data is digitized and encrypted and then they use frequency hopping on top of that to make it even harder to listen in. These systems allow many, many more "channels" than the old systems that only allowed one conversation per frequency channel.
In other words the police scanners you can buy on eBay won't do you any good, they listen in on the old analog radio so all you'll hear are some old ass country bumpkin cops and maybe some fire / rescue. So now days you can't legally buy a system that listens in on the cops.