I have no professional retouch experiene but I have been doing freelance illustration and graphic design for a few years. I have some experience working with photos in concept art for computer games. I'm going to apply for a retouch position at a furniture company and I need to create some pieces for my portfolio. Now, even though using a photo without permission might technically not be legal, how much does potential employers honestly care? It's about showing my skills as a retoucher after all, not in photography. In the concept art field my experience is that people don't give a hoot as long as you have the skills. My impression is that it is a much more delicate issue in retouch - my best guess is that it's because many retouch artists are photographer aswell and you tend to look after your own kin.
It's hard to find good quality images on the free image sites unfortunately. Input appreciated!