You can't because it is a 4th Generation Pokemon available in Pokemon Diamond and Pearl.
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well i think you have to trade it from Pokemon peal or the esiet way to do it is migrate tangella him or her from Pokemon leafgreen then evolve him or her into a tangrowth done.
tangela doesnt evolve in leaf green you have to transfer it to diamond and pearl in order to get tangrowth
Pokemon #465 is Tangrowth
Yes you can trade leafgreen version Pokemon to another leafgreen version.
On Pokemon Pearl's title screen, if the Pokemon LeafGreen game is in the GBA slot, click on "Transfer From LeafGreen" and you can choose six Pokemon from LeafGreen to transfer to Pearl, but be careful-you can't trade them back.