Here's a couple of sites for vehicles used by UK Police forces: http://www.ukpolicevehicles.co.uk/

http://www.ukemergency.co.uk/

And here's another one, which also includes police vehicles used in other countries, including USA, Belgium, France, Germany, Netherlands, Italy, Spain and Bulgaria: http://www.emergency-vehicles.co.uk/ However to see the photos on this site, you must become a member (which requires a membership fee).