You can use Southwest Airlines across the U.S., but they have very limited international service to Mexico, Jamaica and a few Caribbean islands. As the name suggests, the airline is absolutely strongest in the south and west of the US, but many routes also go to the east coast.
Always buy the cheapest tickets on the airline's website , www.southwest.com, and it practically doesn't matter how far in advance you book your flight. The average price of a one-way ticket is about $80.