You could hop on ebay and bid on some anatomical skull models (which run between $20 and $80).
Or, you could just download these complete reference photos prepared by jimiyo exclusively for the Go MediaZine. They’re awesome! Inside there are 15 high res skulls at different angles and cut out from the background. So get your Wacom tablet set up and spend an evening drawing over these reference photos. It’s fun and really great practice.
Also an update: Thanks in part to your votes, jimiyo is in the final five over at the DBH competition! That is really an accomplishment. The votes have been reset so make sure to say thanks for these great resources with another vote. Good luck jimiyo!