If you're looking for images to retouch, I highly recommend Editable Photos. It was created by several photographers here on MM and they have an extensive library of raw files, both free and pay, but credits are really cheap.
There's also http://www.digitalphotoshopretouching.com/
where you'll find 2 sets of RAW Files, but you can't buy in singles, only the full sets.
Other websites to provide royalty free images usable in retouching work are:
The royalty free images however don't come in raw format, just .jpeg and 90% of them had been retouched prior to posting.