Both Juan Santamaría International Airport (SJO) near San José and Daniel Oduber Quiros International Airport (LIR) in Liberia, Guanacaste, offer on-site coronavirus testing for outgoing travelers.
This gives passengers the ability to get tested on the same day as their flight, and it removes the complication of having to find a nearby lab with availability.
At both airports, the tests are conducted by Laboratorios Echandi in a partnership with Morpho Travel Retail.
While walk-ins are accepted, the lab recommends making a reservation at: https://covid.morphotr.com/
First check what test you need to get for your destination country. This article in Tico Times gives a quick guide of the what, when, how and prices.