Lots of aerial photos are available commercially, or for free download. I have aerials of the local area dating from 1921 onwards. The picture at the top of this thread contains about sixty separate images stitched together in Corel Draw. In the uk local councils usually have high res surveys from several periods, and there are firms such a aerofilms who have huge collections. I don't use dems as they aren't available for my area, and anyway, they are very low resolution compared to the models I use. I generate my terrains from local contour maps - see the section on making the images on my page. Its simply a question of getting a section of map which matches the area of the photograph.