I just found this site which give free GPS maps for Garmin GPS units the home page is http://mapcenter2.cgpsmapper.com/
and you can download the Sri lanka road map from http://mapcenter2.cgpsmapper.com/maplist.php?country=208®ion=219&category=2
*********Update - 18 July 2010 ***********
Since there had been problems with downloading the map files from the above location, we have add the relevant files to the lakdasun server. Please check the tread - Free Complete GPS map of Sri Lanka and the world base map for Garmin
for the download locations
Download the installer file (24.7MB) from this page and install it. After the installation the map will be loaded to Garmin MapSource.
And if you guys don’t have Garmin MapSource follow these steps to install MapSource to your computer.
Garmin officially provides a MapSource version update download, which will only work if there is previous version of MapSource installed. If no MapSource software is detected, the MapSource update setup will abort with error message saying that it cannot find previous version of MapSource.
Follow the following trick to hack the setup file so that it can be used to install MapSource clean and fresh without any prior installation of MapSource.
1. Download MapSource software updater from Garmin web site. (http://www8.garmin.com/support/download_details.jsp?id=209
2. Unpack or extract the content of MapSource.exe downloaded using WinRAR.
3. From the extracted files, run MAIN.msi.
4. Then, run Setup.exe. Note: The sequence to execute Main.msi and then Setup.exe is important to avoid “Previous MapSource not found!” error.
5. Garmin MapSource now installed. You have to install Garmin MapSource before installing the SL map
After installtion of mapsource and the map product, if you do not see the Sri Lanka map in Garmine Mapsource program check the following configuration.
Mapsource -> View -> Switch to Product -> Select Sr_Lanka_Routable.
This option is set to "No map" by default and even after intalling the Sri Lanka map, you will see a blank screen unless the map product is set in the above menu option