maximus,
I used to work in the medical field - in the department for neurosurgery to be exact- and thus I had access to various types of specimen that have been scanned in a CT.
The skulls is a real persons skull and has been scannend in a 1mm raster and was later imported into 3ds max via the STL fileformat. It came in at around 2million polys.
I was working on a project back them trying to determine the pros and cons of a virtually modeled bonecap against a traditionally (in situ handmade) cap.
But the details are very complicated so I stop here.

Back to your question: I am afraid that such a model is almost impossible to obtain if you do not have access to them due to some work relations. Or -if you have really need it and have a few grands spare- you could always buy a physical skull model made for educational purposes like here
http://www.anatomie-modelle.de/modelle_ ... QS8_3.html
and scan it in your own 3d scanner here:
http://www.3dscanco.com/products/3d-sca ... rs/metris/
That would also be possible, but still inferior since a CT scan captures all cavities inside the bone as well.
Ben