www.cgtextures.com as far as downloading free images you can manipulate.

Otherwise, figure out with a thumbnail what you wanna make, and go get out your digital camera. This way you'll know ahead of time which angles to shoot from, from what time of day and angle from the sun so that everything matches, etc.

People use various 3D programs to make the basic geometry of buildings and mountains and the like to paint over as well - think of the giant Dracula's castle in "Van Helsing". For that technique, check out any of the mattes done by Raphael Lacoste - brilliant painter - most of his stuff was done for games, but he's recently been hired by a matte painting (actually FX in general - but his job is for mattes) company for movies and TV shows.
Hope this helps-
-Lew ;-)