Southwest Airlines has been expanding its flights to Costa Rica. Southwest first started with a direct flight between San Jose (SJO) and Baltimore (BWI). It recently expanded adding a direct flight between San Jose (SJO) and connecting with Houston Hobby (HOU) airport which opens the connections to many other of their U.S. routes.
The airline has also announced that in April of 2016 it will initiate a direct flight from Los Angeles International Airport (LAX) to the pacific coast of Costa Rica landing at Liberia International Airport in Guanacaste (LIR). The Liberia, Guanacaste airport is the door to the eco-tourism sector of Costa Rica.
You can check the flight schedule for Southwest Airlines to Costa Rica at their flight reservations page.