You can find a free list of police and other emergency service scanner codes at RadioReference.com. You can also search a listing of ham-radio stations, Internet stations that rebroadcast police frequencies and airports.
Intercept Radio also provides a search portal for users to search for scanner codes online. In order to search the FCC databases, specific search queries are necessary to go through the information. When searching Intercept Radio, you can narrow down your search to the type of frequencies you're looking for, as well as the ability to narrow the search down by a particular location, like your city or county.
ScannerMaster.com also provides a way for you to search online for scanner codes. The website provides a lot of information regarding how to search and find certain scanner codes, and you can email the site if you have any questions on searching for particular codes. ScannerMaster.com also provides you with detailed information on the types of scanners that are available and the function of each. ScannerMaster also provides you with the different types of groups you may want to search for, such as a police to dispatch group, a police car to car group, or a fire dispatch group.