To find croagunk in Pokemon Heart Gold, you must first unlock the safari zone editor and be able to place objects. Add marshlands to your safari zone and then place 21 stumps within the Marshlands region. Come back 20~29 days later and croagunk will be found within the grassy regions of the Marshlands. (You won't encounter him every time, so keep walking around until you do).
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Croagunk is not available in the Pokemon SoulSilver/HeartGold games. Actually, it is. You must put forest objects in the marshland area of the safari zone. I don't know how many though.
pastoria great marsh ( not too hard to find )
Well at the Safari Zone you should be able to find him in there somewhere..
You can find it in Pokemon Platinum, Pokemon Crystal, Pokemon HeartGold, and Pokemon SoulSilver.
Find and then catch