We spent 5 weeks driving in Costa Rica in early 2012 and used the third-party software recommended by Garmin Tech Support with great success. It's available from Sergio Vargas, the IT Director at RSGPS S.A. Sergio can be reached at email@example.com
The download cost US$85.
The purchase process is simple; send Sergio the details of your unit then download the file he makes available. Write the file onto your unit or on an SD card in the proper folder and you're good to go.
Many times, a city name on the Osa Peninsula was missing but the map was there -- just put the GPS' pointer on the map and go.
Costa Rica doesn't believe in posting street names other than on the major highways. Even then, navigating through a town on a major highway takes faith. The GPS was always right -- even when it said to turn on some nondescript dirt road that somehow was part of the InterAmerican Highway going through a small town.
You'll get used to following directions like, "Turn left on road to unpaved road". Just do as you're told.
But...pay attention anyway. The occasional reality check is a good thing.