How do you get Sinnoh Pokemon in Fire Red/ Leaf Green?

Hello,

You cannot get any Pokémon from Sinnoh in earlier games.
Sinnoh was a whole new Region, it was designed and coded a while after FR & LG were already being sold. As for this, for example, a Bidoof simply isn't coded into the FR&LG Pokédex, nor was it into the Ruby/Sapphire/Emerald Dex.
You can think of it this way: For the character named Red in Pokémon FireRed, there is not such a thing as Bidoof. Nor there are Dialga and Palkia, in Red's world. They can't even be, as the world of Hoenn and Kanto never heard of the Sinnoh Region.

You have to keep in mind that Pokémon games are in one permanent state, the state which was being coded into the Cartridges in 2004 (If memory serves me right).

Anyways, you could think this: "And why would Kanto Pokémon be in Diamond and Pearl, then?". That is because Kanto already existed when Sinnoh was being created. And for the games to really follow up to each other, Sinnoh's citizens would have to know the Pokémon that already existed and were found, right?

This is the same with any later game. For example, the Unova citizens of Black&White couldn't know that in the future X&Y would be made, which was going to get Kalos as Region.


This is what I could think of to help you, and I hope you understand now.

Have a nice day,
And greets from Xanthos.