Great question! Here are some ideas for where to look:

• The Tico Times English-language news website does a weekly roundup of events happening all around the country so that’s a good place to check. Usually they post things closer to when the event happens, so check in around March before you go. http://www.TicoTimes.net. A lot of times, they share happenings in CR on their Facebook page too.
• The tourism board has an events page that updates for the upcoming month (requires some Spanish) http://www.visitcostarica.com/ict/paginas/eventos/calendario.asp.
• This website (http://costarica.com/culture/holidays-festivals/) has a good list of holidays in Costa Rica. Many holidays have parades or festivals if you’re interested in cultural experiences.
• A new website launched recently for this exact purpose- http://www.puraeventscostarica.com/. You can search for events by province.
• As for farmer’s markets, all those towns you’re going to have them, we believe. This site (http://upwardspirals.net/organic-directory/) has a list of the organic ones in Costa Rica at least.