After beating the elite four and becoming Pokemon champion you should arrive in pallet town prof oak should come up to you and should check your pokedex if you have recorded 60 Pokemon in your pokedex he will admire your accomplishment then take you into his lab and upgrade your pokedex now you have the national pokedex this means you can own and capture Pokemon from the johto and hoenn regions.
You have to trade with firered or leafgreen to get the national pokedex.
trade
see every Pokemon
beat the Pokemon league.
beat the elite four once
never leafeon is NOT in the kanto pokedex anyways there is no national pokedex
The national is the National Pokedex. This pokedex allows you to catch/see new pokemon. You'll also get to go to 4,5,6, and 7 island.
get firered or leafgreen and migrate it to pearl after national pokedex
you find in snow point temple after you get the national pokedex
trade it from ruby sapphire or emerald after getting the national pokedex.
You go to route 228 after you get the national pokedex and migrate Pokemon from leafgreen
in peral/diamond you have to get the national pokedex in firered/leafgreen you can catch it in the later stages of the game