In the mangroves of Sierpe. A really awesome thing to behold is a row of these small bats, usually about 7-10 hanging below a tree. Saw them in Corvovado and Tortuguero as well.
@AD: Thanks. I'm happy with the result as the photo was taken from inside a tiny boat
@Chlidonias; Luckily, if you even get to visit Costa Rica, these bats are not really rare. I saw them in the mangroves of Sierpe and the Corcovado National Park on the Osa Peninsula (in the southwest), in Tortuguero National Park (in the northeast) and in the Sarapíqui area (central area). They are always near water, and most guides will be able to find you some as they often roost in the same trees.