Check here for bus information: www.thebusschedule.com

You can take a bus from the bus station in Alajuela to the town of Puntarenas, get the ferry there and take another bus once you reach the other side.

The bus station in Alajuela (this is where the airport is located) is known as La Radial. (If you don't know how to say it, just write it down and hand it to the taxi driver.) You can take a taxi to the bus station - it's about 5 minutes away. It is a newish station with a little convenience store, clean bathroom and a security guard.

If you check the bus schedule for buses from San Jose to Puntarenas, add 30 minutes to the departure time from San Jose and that's what time it will arrive in Alajuela.

Alternatively, you can take any bus that is going to Canas or Liberia and get off at the entrance to Puntarenas (the center of town is a few miles off the highway) and get a taxi to the ferry from there. The advantage of this is that buses for Liberia leave about every hour from San Jose.

No buses originate at this bus station; they just make a stop here picking up passengers. You will pay the driver in colones.