Totally rad used to make a texture plug-in (or action?) for photoshop, think it was called something like dirty pictures? That was quite good.
When I'm out and about I look out for t for textures and skys and keep them for later use, rusty metal works well. There used to be a lot of places with free textures for photoshop, not sure if it's as popular now. I havent done one for ages.
You can make some good background looks using the render clouds filter.