Just as an aside..."Looking for a tornadus... in return for a thundorus"...don't do that. Borrow someone's Tornadus as long as you give it back (they can't be breeded and they might want to keep it, but if not, bonus), then use it in the post-game to catch Landorus (both are needed in your party).
We might be able to work something out here. What would you send me as an insurance policy to borrow my Tornadus? I don't mean an easily caught L2 Pokémon from Route 1, I mean something more valuable that you've trained that I can care for (or even breed) while you get your Landorus; your fully evolved starter (anything but Emboar) would be perfect as I can breed it. There might even be a baby starter in it for you when all is said and done (unless you chose Tepig, then that idea is screwed). I'd hope you're willing to return the favour afterwards as well, perhaps? :)