You'll have to wait. For MS Flight sim there is a very strong developer community that loves to make mods, and many of them are free. But with new game features comes a whole new method of designing aircraft, it will be a few months before those are out there. That is, assuming FS 2004 aircraft are incompatible with FS X. If they are compatible, they're everywhere. Just search for Flight Simulator 2004 aircraft and you'll find many developer sites. All you do is drop it in the aircraft folder and you're good to go. Also, there are professionally developed add-ons that will (most of the time) be much higher quality than the freeware downloads, and those typically cost around $20.
I don't think the developer community will be as strong this time around because of the SDK being in the expensive package. Is that the only place you can get it? Last time around it was free for download, you just needed discreet gmax and some plugins. Does anyone know?