you have to go to Pokemon indigo i think and before the battles theres a shop and i think u buy it from there
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safari place
You have to use item "Revive" or "Max Revive" or you have to go to the Pokemon Center and talk to Sister Joy. There are also places and people in certain locations that can revive your fainted Pokemons.
You can not buy a Max revive in Crystal. You must find them in the wild.
route 214
Lilycove Department Store, Petalburg City