Get the national pokedex and go to the Great Marsh. trust me i got this info from Bulbapedia.
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You can find some Kangaskhan in the Great Marsh, so be sure to look from the second floor of the Observation Deck of the Great Marsh to find areas where Kangaskhan can be found.
no, but if you want to get baby kangaskhan you need a ditto leave it at the day care with a kangaskhan and all done hope i helped
safari zone
Kangaskhan can be found rarely in the Safari Zone north of Fuschia City.
you find kangaskhan in area 2 of the safari zone in Pokemon leaf Green ... not sure in fire red sorry