they come out mostly at night.
You go right from Pewter City.
Mt Moon is only one floor in Heart Gold/Soul Silver.
Mt. Silver need rock climb + surf + waterfall Moltres
You can find clefairy in mt.moon, but it is very rare.
Clefairy, Jigglypuff, Nidorina and Nidorino
Clefairy, Jigglypuff, Skitty, Nidorino, Nidorina.
Mt Moon- 1F+2F, 5% chance of encountering a level 8 Clefairy. Safari Zone-Meadow (night)- Level 18 Clefairy. Safari Zone-Meadow (3 Peak objects)- Level 42 Clefairy.
Clefairy,Nidorino,Nidorina,Jiggypuff.That's all I can think of.
Whitney uses Clefairy and Miltank in Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver and in the anime she has a Nidorina, a Clefairy and a Miltank.
Nidorino & Nidorina, Clefairy, and Jigglypuff. There could be others, but these are the ones I know for a fact :3
as far as im aware the only way to get it is trade it from gts or pokemon diamond, pearl or platiunm
If you go to Mt. Moon, you will eventually exit on a place where there will be a small pond. If you go there at night, supposedly there will be a group of clefairy that encircle the pond. Talk to the people living in the house near that pond for more information.