It is perfectly legal to buy and sell human skulls/bones. Most were imported from china and india in the 1980s. Since then the flow of product has slowed so prices have sky rocketed.
If you are looking for a real one, it is illegal and actually kind of immoral. You'd have to smuggle one in from another country. For a replica, check out any costume or party store or online.
I don't thing is anywhere in the world a store where you could find human skulls but I have seen some days ago such things on e-bay and on another link.
Im sorry but all of these people are 100 percent WRONG. The sale and posession of human remains in the united states is only regulated in terms of native americans. I myself have 14 human skulls including a tibbetan tribal skull.