How do you get a aerodactyl in Pokemon heartgold?

Answer 1

There are two ways.

1. Use an ARDS, turn on (or enter) the cheat "catch trainers Pokemon, then steal Lance's. Or,

2. Take an old amber to the Pewter City Museum above the gym. I think you may be able to find one by the museum. I will check to be sure. EDIT: I didn't find one. I am pretty sure that there should be an Old Amber somewhere in Kanto.
You need to get the old amber fossil. U get it by smashing rocks at the ruins of alph.
look around for a fossil at Ruins of Alph, it's relatively rare so keep trying. After that go to the Museum in Pewter City to get it revived!