I guess you mean the heightdata? Without any real knowledge of Skyrims terrains system, but the steps should be like this. Experiment!
Find the coordinates for the location you want to import to Skyrim.
Plot them in http://www.gdem.aster.ersdac.or.jp/search.jsp site and download the correct DEM (Digital Elevation Models). You need to register but the heightmaps are free for educational purposes.
Convert to .raw with the correct specs. This can be done through http://www.usna.edu/Users/oceano/pguth/website/microdem/microdem.htm which is free.
Import into Skyrim, however, this may be a real problem as the Creation Kit is a bit incomplete at the moment. In this forum section some 10 threads discuss importing of heightmaps and the problems associated with it!
Good luck!
What would be the point may I ask? (I am new so go easy if this is a dumb question)
Well, if you take a not so well known area in, say, the middle of Russia, Alaska or northern Scandinavia you can get realistic terrains for your fantasy world.