Since only Pokemon found within the Kanto Region Pokedex can be found within this game, this means that there are only three different fossils players can find in order to revive and obtain the Kanto Prehistoric Pokemon. After defeating the Super Nerd at the end of Mt. Moon, you will be given the choice of receiving either the Helix Fossil (which can be revived into Omanyte at the Research Lab on Cinnabar Island) or the Dome Fossil (which can be revived into Kabuto at that same Research Lab).
Players can also obtain the Old Amber (which can be revived into Aerodactyl) from the scientist in the back of the Pewter City Museum, only accessible after obtaining the Chop Down Secret Technique. On top of this, players also have the chance of finding any of these three fossils as a Hidden Item located just in front of the large middle crystal on the second floor of Cerulean Cave.