WetCanvas, a popular forum for artists has a "Reference Image Library" that is full of photos supplied by members copyright free.
http://www.wetcanvas.com/RefLib/
You might have to be a member to access it though, but membership is free.
There's a catch however, just because a photograph is uploaded to a copyright free website does not mean there is actually no copyright on that image. There's no way to know if the uploader actually owns the copyright and has the power to give others permission to use the image. At the very least you should do a reverse image search before using an image you get off a copyright free website to check if there actually is copyright on that image.