Replacing surface textures in FS2004

I am brand new at this. I want to build an airstrip at a remote location. I am studying the instructions for AFCAD2. I assume this is the program for creating runways. First I want to change out the default textures of the airport area with a grass texture, like you see in Emma Field, Skykomish,Darrington, ect. What program would be best for doing this, and where does one get the textures? I would appreciate any help or direction here.Thanks in advance.