My friend keeps bugging me about how to get an Aron because she thinks they are sooo cute. So I came looking for the answer on here. ( I looked in the location and the questions catagory but there was nothing for Heart Gold and/or Soul Silver. )
If you place 24 Peak objects in Rocky Beach, Aron should appear.
Get the national pokedex and go to safari zone and one of the areas just add rocks and maybe they will appear I'm not sure.