Searching the internet, found One guy who managed to create custom DEM files for garmin GPS. He said that the original DEM data came from ASTER, which provides it for free covering the whole world. ASTER GDEM data is available for download at http://www.gdem.aster.ersdac.or.jp/index.jsp.
He processed and prepare IMGs contain DEM for Indonesia, ready for your Garmin GPS which support 3d map. These DEM IMGs covers the whole country of Indonesia, tested with GMPC, Colorado, and Oregon. Logically, other similar Garmin products should be able to use it also.
More details are available at http://anonymus-wolfy.blogspot.com/