Out of 125 sites dedicated to police scanning, the best frequency source is Radio Reference, according to Zip Scanners. It also lists CityFreq, Intercept Radio, and ArtSciPub as valuable sites, as of 2015. Most of these resources are collected by enthusiasts and should not be considered official sources.
Radio Reference provides a searchable database of frequencies and transmission modes for communities across the United States, notes the website. It provides a map-based interface, allowing selection of the state and county to search. The database may also be queried by metro area, city or ZIP code. The data is retrieved at the county level. It includes broadcast frequency, license ID, and a text description of how the frequency is used, such as by local sheriffs, by fire dispatch and so on.
Each of the top sites has a slightly different focus and methodology for searching. The CityFreq database includes international phone prefixes and IP addresses. A county-level map interface appears on Intercept Radio. Once a county is selected, the database reveals a frequency table with links to transmitter call ID data from the FCC. A database of radio repeaters is maintained at ArtSciPub. Lookup for these repeaters is by state, ZIP code or call sign. Repeaters generally do not carry local police radio activity.