You can't, you have to catch him in 102, there are none in 103, at least not that I know of, check your Pokedex.
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You can catch her on route 103, really rare though so look hard! :)
It is Altering Cave which you can enter only after you beat the Pokemon league.
its very easy :) hafter beating Pokemon league go to slateport city and head at route 103. go to that lake at this route and what is that?IT'S A NEW CAVE!!!! and the cave its named ALTERING CAVE
go north from littleroot town .. just keep going
First, you go to oldale town.Then, go straight to route 103 and last there is May