There are DEM files for nearly every location of the world.
You need to find a sample .DEM file - just try google...
Next is to find a converter from .DEM to a file format which is supported by Maya.
There are differend files in different quality and resolutions.
These files should work for a helicopter/airplane game but maybe you need some detail maps for FPS games.
These terrains contain only the landscape. - no trees, no destroyed buildings, no roads or railroads. Simply pure landscape with hills, mountains and such.
But you can fill a level with a terrain and everything else you need.