Step 1: Aquire your skull
1. Check your local taxidermist. Many times they will have small animal skulls in the freezer, such as coyote, beaver, fox, raccoon, bobcat ect. depending on where you live. They may charge you a few bucks for one, or they may even just give you a skull or two at no charge.
2. You can occasionally find raw skulls on eBay. This has the potential to get a little pricey and possibly a bit smelly, because the skull will have to be shipped to you. When searching for a specimen on eBay, type in "taxidermy skull" and search in "all categories".
3. Road kill. Use this as a last resort, as the skull will most likely be rotten and very possibly damaged.