you have to go to the old rich guy's mansion and go to his trophy garden, after talking to him a couple of times, he will tell you a story about how an igglybuff ran up to him in his trophy garden, from there, he'll send out his butler who denies that it isn't there (probably the trigger to putting some in there, lol) but then go to the trophy garden and run around and you'll find one or two, then keep it happy, dont let it die, put a soothe bell on it, etc. it will eventually evolve into jigglypuff
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In Pokémon HeartGold, you can find Jigglypuff in the Safari Zone at Levels between 15-17 in the Meadow and Swamp areas additionally you can find it in Routes 3 and 4.
at trophy garden Baclot will say a jigglypuff gave him a smooch on the cheek.It will take a while ,but you will find one eventually
You can find a few on route 3.
You can only get it from breeding it from Jigglypuff or Wigglytuff
You have to breed a (female) Jigglypuff or Wigglytuff.