Old Amber is a random, rare find in the Underground. It's not actually found in any specific spot, like you seem to think. It can be found anywhere, it's just rather rare. :)
At the beginning of the game before getting the national pokedex you can get: Skull Fossil - Cranidos *Exclusive to Pokemon diamond Armor Fossil - Sheildon *Exclusive to Pokemon pearl After receiving the national dex you can also find: Helix Fossil - Omanyte Dome Fossil - Kabuto Old Amber - Aerodactyl Claw Fossil - Anorith Root Fossil - Lileep
Find a fossil underground. I did.
Root Fossil Claw Fossil Helix Fossil Dome Fossil Old Amber Armor Fossil Skull Fossil Rare Bone
you can find every fossil there is in all the games: Skull fossil: cranidos, Shield fossil: Shieldon, Root fossil: Lileep, Claw fossil: Anorith, Helix fossil: Omanyte, Old Amber: Areodactyl and a Dome fossil: Kabuto OBS you can only find all of them AFTER you get the National Dex. Until then, you can only find the Shield fossil if you have Pearl, and the Skull fossil if you have Diamond
You cannot get Cranidos in Pokemon Pearl unless you trade to get the Skull Fossil from a friend. You can only find a Skull Fossil in Diamond.
in eterna city
at the oreburgh mining museum:)
Get a old amber and take it to a guy in the museum in Petwer City,
Old Amber is a kind of Pokemon fossil. It is the same in every game, including Pokemon Pearl. Thus, being a Pokemon fossil, Old Amber can be restored into a Pokemon by the technology of the Oreburgh City museum. Simply leave the Old Amber with the strange man in the city museum and return to find a restored Aerodactyl on level 20.
The same way you get a skull fossil on diamond, dig in the underground till you find one.
You need to go in the underground and find an Old amber or trade it with someone.
Helix fossil gives you Omanyte Dome Fossil gives you Kabuto Old Amber Fossils gives you Aerodactyl Root Fossil goves you Lileep Claw fossil gives you Anorith Skull Fossil is Cranidos Armor Fossil gives you Shieldon