Yes, you can always at least find a mini market. The bigger stores are Automercado and MegaSuper.
There are so many delicious specialties you should buy in Costa Rica. These include....
Guayabita is a popular candy here in Costa Rica. The outside is a chocolate coating and the inside is a guava flavored jelly.
Friend plantains are a tasty snack like potato chips. Usually, they are seasoned with salt.
Yuca chips are very similar to potato chips. They come in all sorts of different flavors. I personally love the sour cream and onion flavor.
You can buy pre-made tamales at almost all grocery stores. Just heat them up in boiling water, peel open the banana leaf, and they will be ready to enjoy.