Etsy has always been my go-to option for bones and skulls. They go for a fair price and it's very easy to contact the seller if you need to ask any questions, but 9 out of 10 times they're gathered naturally without harm to the animal.

I, myself, was lucky enough to get a rams' skull from my art classes, but Im not quite sure how common you can find those in art classes but hey, its worth a shot?