In Generation I and Generation III the Old Amber can be found in a back room of the Pewter Museum of Science. You need the HM Cut to get there. In order to revive it you must take it to the laboratory on Cinnabar Island.
In Diamond, Pearl and Platinum an Old Amber can be found by mining Underground after obtaining the National Pokédex. Old Amber can also be found at the Ruins of Alph in Heart Gold and Soul Silver by using Rock Smash. The Old Amber can be used to revive Aerodactyl at the Pewter Museum.
Fossils are given away once a day at Twist Mountain in Black and White, but the fossil that you get everyday is at random. The Old Amber is one of the fossils that can be obtained this way, and can be revived at the Pokémon Gym in Nacrene City.
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