How do you get a national pokedex in Pokemon LeafGreen?
see every pokemon in the kanto region then visit professer oak
A national pokedex is usually gained late game, the national pokedex is an upgraded pokedex which records information about Pokemon around the nation, including Pokemon from previous generations, a national pokedex also allows Pokemon to evolve into forms which have been discovered in different games or generations i.e (in Pokemon Firered and Leafgreen, it is only possible to evolve golbat into crobat after acquisition of the national pokedex).
Unfortunantely, it's impossible to catch ANY Pokemon from the National PokeDex in Ruby and Sapphire. You have to trade with FireRed, LeafGreen, Emerald, Colosseum, or XD to get Pokemon from the National PokeDex. If you trade with the first three games, you'll actually get the National PokeDex in Ruby and Sapphire.