The Skull and Cover Fossils are purchasable once each in Sun from Konikoni City, while the Armor and Plume Fossils are bought in Moon. If you want more Fossils than these, you'll have to trade them over from Ultra Sun and Ultra Moon, where the rest are purchased once each in Konikoni City and available infinitely from Poke Pelago's treasure hunting.
All Fossils are available in this way except the Old Amber. Aerodactyl is a gift Pokemon in Seafolk Village.
Fossil (Bulbapedia)
