Since people tend to clone pokemon a lot, a darkrai isn't as rare as it could be, so you can probably just try to get someone to trade it to you, to fill out your pokedex, and then if you want a real one, try trading on the GTS with a decently rare pokemon, maybe even palkia. Since their are millions upon millions of players in the world, bound to be atleast one with a cloned darkrai who wants a palkia.
Currently looking for ho-oh, and latios. Can trade almost any other pokemon, or a ev trained non legendary pokemon with personality of your choice. Pm me for information or offer.
FC: 2921 5682 9359, and my name is Steven.