Where do you go to trade with Pokemon Fire Red and Pokemon Emerald?

You have to get the ruby and the sapphire from the sevii islands. First you need to get the national dex which requires you to beat the Pokemon league and have had at least 60 Pokemon on your pokedex. After you do that talk to Oak and he gives you national dex.
(The ruby is under Mt.Ember where those two rockets were blocking the way and after getting national dex, have conveniently moved)
beat the elite four