it is a very long list, so please check serebii.net for a full list of the Pokemon in leafgreen. Thanks! :D
either you hack it or you unlock all of the areas and find every single Pokemon
YOU NEED TO FIGHT ALL POKEMON AND YOU WILL COMPLETE THE POKEDEX. AND THAT WILL HELP YOU TO GET THE NATIONAL POKEDEX BY TALKIN TO PROFESSOR OAK.
If you want to get all the Pokemon in LeafGreen, you could purchase a GameShark for your Gameboy Advance SP or Nintendo DS.It contains all the Pokemon for the games LeafGreen, FireRed, RubyRed,...
There is Zapdos, Moltres, Articuno, Mew and Mewtwo.