This free Garmin map is created using data from OpenStreetMap.org on MacOS X and I use it on my Garmin Dakota 20. The download is a 7-zip file and contains the map in .gmapi format that will work with Garmin Basecamp. You will need the Garmin MapManager and MapInstall software to install it. If you don't have software for 7-zip you will need one as MacOS does not natively support the format. I like Keka, but there are others that will work too.
It will probably work on Windows if you copy the .gmap folder to %program files%\Garmin\BaseCamp\Maps or where ever the maps folder is on your computer, but it has not been fully tested. Ensure that you read Garmin manuals for instructions on installing maps as I can take no responsibility for anything that might go wrong. If you find any errors in the map consider joining OpenStreetMap.org and contributing to the map.
