I went out looking for sheds with a friend out in the desert to the NW of Kingman, and he found a huge desert ram skull. That was about 15 years ago, and it was legal to keep them at that time (and maybe it still is, I don't know). When sheep get sick, they go to the small springs in the desert where they sometimes die and leave behind their bleached bones and skulls. You might stop by the AZGFD in Kingman and ask about the current laws on that, and inquire as to where is a good place to go to find sheep skulls if it's still legal.