Matte Painting is not that complicated... It just needs a little time and patience like any other craft :)
There are different approaches you can take. Usually a layout through speedpaint and / or 3D helps you get your perspective right and a working composition, the 3D lighting of a rough 3D layout also helps later integrating the photo elements. Afterwards using more elaborate/ finished 3D elements and stock footage helps give it the photorealism you need. (Gnomon workshop has great demo DVDs on the subject)
For stock images... well a good digital cam can't be beaten but the web is also full of sites like "flickr" that have zillions of available material. You just have to be carefull to use only parts of the images as not to break the owner's copyright. If you want to use a full image "as is"... then asking the owner for permission is probably the best thing.
hope this helps a little :)
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