Looking for a cheap flight in a far-away place? Most low-cost carriers (like Southwest in the US) don’t list or sell their fares through aggregator websites (like Expedia and Travelocity) — you need to book directly with the airline.
To find out what low-cost carriers fly between any two given cities, check out AirNinja. They let you enter in your starting airport (and a destination, if desired), and show what low-cost airlines have departures from that airport and where they go, complete with links out to the airline’s own site for booking. And it’s free!