You have to go to the safari zone in west Johto. According to Serebii, you must use the area customizer and have one of the zones be "Marshland". You must then place 21 forest themed objects in the area. Then, wait 20 days (Sighs). You can then find croagunk in the tall grass. You can also look for it at the global trading station, or trade it with a friend who has diamond, pearl, or platinum.