Here's why:
* Fossils are found through exploration: You obtain fossils by digging them up in specific locations in the game, typically in the "Fossil Maniac's House."
* They are used for revival: Once found, fossils are brought to a specific location (the "Fossil Restoration Center" in Soulsilver) where they are revived into Pokémon.
So while you can't purchase fossils, you can definitely find them and use them to add powerful prehistoric Pokémon to your team!