Upvote:4
A few links:
Lonely Planet page about bus travel in Cost Rica
A PDF with bus schedules from all over the country including some information on how to use the buses. It is half in English, but some information is only in Spanish: 'Todos los dias' (every day) and 'Cada xx min.' (every xx minutes) and the names of the days: Lunes a Viernes (Monday to Friday), Sabados (Saturdays), Domingos (Sundays) and feriados (holidays)
I never planned or booked any of my bus trips in Costa Rica, I just went to the bus station. In the capital San Jose there is no central bus station, you need to find out where your bus leaves from. The PDF brochure mentioned above has the addresses of the bus companies.
The only thing I remember from buying tickets at a bus station in Costa Rica was that I yelled at an older woman for skipping a long line. It turned out that pensioners have the right to skip the line and she excepted my apologies.