http://www.policecarmodels.com/ is, by far, the most comprehensive police car model dealer. In addition to vehicles, they have a great selection of lightbars and decals so you can customize your own units. All the Crown Vic's have the full wheel covers because it's a lot easier to disguise a full wheel cover than make one. For example, here's one of my Busch models that I've painted so the wheels appear to have just the hub covers:
Unless you hold it right next to your eye, you can't tell the wheel covers have been painted. A little flat black paint and a fine brush is all it takes.
Of all the Crown Vic's, the most accurate Police Interceptor version is actually the cheapest, made by Model Power. It's the only one that has the correct dual exhausts and wider wheels and tires. The only problem is they only come with Vision light bars and they aren't very good. I get the slick top models and then add my own light bar, decals, spotlights, pushbars, and antennas.
As you can proabably tell, I'm a retired cop so I probably get a little more anal about police cars than the average modeler.

Fire trucks I'm not so good with since I always ran out of buring buildings. Boley makes a very nice line of fire trucks and I have a few for the town fire station. If you go to
www.Walthers.com and search on fire truck, you'll see there are a lot of models available. There are probably websites like PCM devoted to fire trucks models also so a Google search would turn up something.