The Federal Communications Commission has designated frequency 155.370 as strictly for law enforcement use. Most county sheriff departments utilize a frequency somewhere in the 155. range. The larger the department the greater number of frequencies will be used. For example, Cook County including Chicago, uses 25 separate channels.
RadioReference.com carries a nationwide database of all publicly used radio frequencies, including law enforcement, fire and rescue, maritime and air to ground channels. The site offers a county by county listing for each state that details channels used by sheriffs and local police departments. It also features a list of federal and statewide emergency frequencies.