I went and looked at some work a friend of mine was working on a few years back for ORBITER. One of his links indicated you can get high quality tiles of parts of countries here.

http://glcf.umiacs.umd.edu/index.shtml

This is how you can build high quality areas on a lower quality map to save space. Also allows addons from the community.

Hey Dr. Sid did any of those links help? If needed I can continue the search. However I hope all that you have gotten so far will be enough to build the sim no?

Good luck!