Area codes are often listed in the front of telephone books. You can also get the area code for a particular city by looking at the entries in Martindale-Hubbell, the AALL Directory & Handbook or any other national directory. Finally, you can always call Information (411) or the Operator (0) and just ask.
To find the location of an area code, search AreaCodeLocations.info or use the Lists posted at AreaCode-Info.com.
Lots more information about Area Codes is posted on AreaCode-Info.com.