As noted in the Terrain Awareness article:
The default data source is NASA's SRTM data. We use a specially formatted version of the RAW data and don't really have a public viewer for it. It is a pretty commonly used data source due to its worldwide coverage.
The data was taken back in 2000 so you will have to be careful with earth that has been modified since that time.
You may be able to check some of the data out here: