Metropolitan Area Airport Codes
Metropolitan areas might be served by multiple airports. The IATA assigned easily readable airport codes to those regions which include multiple airports. The New York City area for example is served by John F Kennedy IATA: JFK, Newark Liberty International IATA: EWR and La Guardia IATA: LGA. These airports are aggregated into the NYC airport code.
Some booking websites allow users to search airports in the vicinity of the given airports. Those that do not may be tricked into extending the search by using the larger area airport codes as listed below. If you book international flights that connect through a major hub you should decide if an airport change is feasible, i.e. flying into the Buenos Aires Ezeiza International airport IATA: EZE and connect to a domestic flight from the local Aeroparque Jorge Newbery IATA: AEP. Smaller domestic airports are more likely to be served by low cost airlines but that might also force you to re-check your luggage. Ask you airline if they allow interlining tickets - checking your luggage through to the final destination on multiple tickets.
List of Airport Codes
Airports are listed by decreasing importance for international travel. Some flight search engines might include not all airports into their queries.