I was in the midst of a long response with technical explanations when the system crashed.
I'm not going through that again. You need a scanner which will follow "trunked" systems, which includes most fire and police agencies now, where the frequency constantly changes.
I am well satisfied with a Grecom PSR-800 which I bought several years ago for $300. Most police and fire agencies in the United States are pre-loaded in the radio.
It is somewhat complex to set up, but fairly easy to operate. See the owner's manual at
http://www.greamerica.com/download/m...800_Manual.pdf
The company also produced a similar scanner under the Radio Shack label. For more information see
PSR-800 - The RadioReference Wiki
Possibly you can find a "trunking" public service scanner on eBay. Good Luck.