You can't.
Chat with our AI personalities
some show up in white forest
only if you trade them from red, blue or yellow
use the time capsule upstairs in the Pokemon center
the only way to get one of the starters of Kanto on pokemon crystal is when you completely finish your Pokedex, professor oak will give you one o the Kanto starters.
You cant get a kanto starter on Pokemon sapphire but if you have Pokemon emerald then you complete the pokedex then you get a choice of starters from the johto region. I hope this helped.:)