[sliverstorm]

This tutorial will direct you to download DEM data for the US:

http://www.gpsfiledepot.com/tutorials/how-to-create-garmin-t...

The map viewer you use:

http://viewer.nationalmap.gov/viewer/

Info about the elevation data you get:

http://nationalmap.gov/3DEP/3dep_prodserv.html

The data is kind of buried, but I went through this process for building maps and it appears to be very high quality.