Posting this might knock the site down. However, the University of Texas has put up some pretty sweet models and info on various animals. And feature downloadable .STL files of SKULLS!
If you've never dealt with .stl files you can open them with Meshlab:
And export them from Meshlab into .obj
Been playing around with them and they are really fun to sculpt on. I'll post some of those sculpts soon, It'd be cool to see other peoples too.
Edit: haha fuck... here's the link - http://www.digimorph.org/resources/STLs.phtml