Anyone know where I can get some or where I can get heightfield maps for real geographical structures on Earth?
You can go to seamless.usgs.gov to get DEMs, and then use a free program called 3DEM to convert them to Terragen's format.
MOLA is the Mars Orbiter Laser Altimeter. http://mola.gsfc.nasa.gov/
3Dem http://www.visualizationsoftware.com/3dem/ will read, overlay, crop etc MOLA and .dem data, and export it to TG .ter format.
Or USE L3DT an awesome App. At http://www.bundysoft.com/L3DT/