Hi jmac,
Those types of products can be very hard to find in Costa Rica. We know of a couple of outdoor gear stores in the San Jose area but have no idea if they have freeze dried food. The closest one to Alajuela is Mundo Aventura (http://www.maventura.com/) in downtown San Jose. You’d want to call or contact them first to make sure they have it, but honestly, the easiest thing is probably just to bring it with you.
We just visited Sirena on a day trip and it was fantastic. Hope you have a great stay.