[GEN VI] need legendary Pokemon to complete the Pokedex.

I need help completing the Pokedex. I do not have access to all legendary Pokemon to complete the Dex on my copy of OR. can someone help me? The Pokemon i do not have yet are;
Zapdos, Moltres, Articuno, Lugia, Dialga, Manaphy(optional), Thundarus, Tornadous, and Xerneas. Volcanian is also optional though.
I can't promise trading good Pokemon for these legendary creatures, but i thank those who help me .

You can get Tornadus in-game by soaring with Castform in your party over Fortree City. There should be a storm cloud that you will need to enter to battle Tornadus.

i actually got most legendaries, it's okay though, but thanks for the tip.