In Mt.moon, but be patient, they are not common
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Breed a Clefairy with a Jigglypuff.
It will evolve jigglypuff into wigglytuff, clefairy into clafable, nidorina into nidoqueen, and nidorino into nidoking.
Once you ask the Copycat in Saffron City to get her Clefairy Doll back Talk to the man inside The Pokemon Fan Club.Then he will give you the Clefairy Doll go outside.Then Steven Stone will talk to you he will say that Latias(HeartGold)or Latios(SoulSilver).Finally Latias or Latios will roam Johto or Kanto.But sorry,you can't get Latios without Cheating,Hacking or Trading. :(
Clefairy's Egg Group is Fairy, so any Pokemon with the same Egg Group can breed with Clefairy, including Ditto, which has its own Egg Group.
Clefairy cannot evolve into Clefable by leveling up in Pokemon FireRed. The only way to evolve Clefairy is by using a Moon Stone on it.