you need a cheat for it.
You can catch them on Iron Island once you have the National Pokedex. You get this by seeing all the Pokemon on the Sinnoh Pokedex.
Get the National Pokedex by seeing every Pokemon in the Sinnoh Pokedex
You can't.
You get the sinnoh one basically after starting the game, the you get the national pokedex when you´ve completed the sinnoh pokedex (by seeing all the Pokemon in sinnoh)
By seeing all the Pokemon in sinnoh and going to rowen's lab.
To get your second (national) pokedex you have to complete the first (sinnoh) pokedex by seeing all the Pokemon in sinnoh. when you have, go to the professors lab in sandgem town and show him the pokedex, then professor oak will come and give you the national pokedex.
use an action replay
get the national pokedex and talk to Bebe after seeing 151 Pokemon
First off, it's National PokeDEX. You do this by seeing every Pokemon on the Sinnoh-dex.
After seeing all of the Pokemon in the platinum pokedex (250 I think) and beating the elite four. you can get the national pokedex which contains all 493 Pokemon
To get the National Pokedex, you must see all of the Pokemon that are in your regular Pokedex. You do not have to capture all of the Pokemon, but merely seeing them is enough, and it doesn't matter what gender the Pokemon is either.
get a national pokedex by seeing every Pokemon in sinnoh