1. As I said, DEM are for eye-candy. You don't need "more resolution." Why not use the DEM already created by Garmin.
2. You CAN build your own DEM (at various resolutions and fixing holes in SRTM data)
3. Extracting DEM from copyright product (especially from Garmin) and using it for something else is "publicly" forbidden*. Take your complains to Garmin.

* So even if such tool is available, it won't be posted