I've used this GPS map personally, works great!

to use this simply get a blank SD card 256 MB will do
and on the SD card create a folder and name it garmin ,locate and copy and paste  put the gmapsupp file in the garmin folder .
that's it ..
 to check the map put the card in your garmin device
and turn it on , when it is started click on the tools or setting icon and
then click on the map icon , next click on the map info icon and you
should see a checked box with the Costa Rica map info after it .
  you will also see your factory installed map info .