you need to migrate it from Pokemon diamond/pearl/platinum.
it is unobtainable in soul silver/heart gold
trade, not migrate
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You can trade one from another game, in soulsilver you can find a trainer with a chimchar, then go to the GTS and evolve it.
You get a Chimchar if you want it as your 1st Pokemon
You must breed a Monferno or Infernape with a Pokemon to get the chimchar egg.
The only way you can obtain a Chimchar in Pokemon Heart Gold or Soul Silver is through a trade. Pokemon Diamond, Pearl, or Platinum are the easiest ways to get one. If this is not an option for you, you could always try the GTS in Goldenrod and hope someone gives a sane offer. But there is no way to catch one yourself. Sorry.
You can get a Chimchar if you trade it from games Diamond, Pearl, and Platinum.