Most rent car agencies are represented in Costa Rica and have offices near both airports.
Some of the most popular are:
Poas - www.poasrentacar.com
Alamo - www.alamocostarica.com,
Hertz - www.hertzcostarica.com
Other agencies are available
Buy insurance - some agencies include it in the daily rate, others don't
Book in advance.
Map from San Jose to Garza
Map from Liberia to Garza