After the game, go to the Great Marsh and search.
About the west side, i'd say.
SINNOH POKEDEX: Riolu. NATIONAL POKEDEX: Kangaskhan.
114 = tangela 115 =kangaskhan i have my Pokemon POKEDEX book
you go to pastoria and into the great marsh note:you can get it from emerald as well
once the national pokedex is obtained, you can find them in the pastoria great marsh. always female, and between lvl 24 and 26
great marsh
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.
you find kangaskhan in area 2 of the safari zone in Pokemon leaf Green ... not sure in fire red sorry
You can find it in the rock tunnel, but it is rather rare.
You can find him/her in the safari zone.(If you beat the Elite Four.)
you need to find it in the safari zone in area1.
Since it's a Kanto Pokemon, it must be traded from FR/LG.