In Route 9, 38, and 39.
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you trade it from another Pokemon gameyou can not get mankey on heart gold or soul silver yoU have to get him by migrating
you should look at serebii.net for that answer.
The route west of Viridian city. You can also get a mankey in rock tunnel
You can catch one at Route 225 or at Route 226 (using the PokeRadar).
You must trade because there aren't Mankeys in Crystal