i'm in your conscience
There is a place, but it was only activated by an event that can no longer be accessed. If you still need a Darkrai, you could browse the trade shops in Trade Corner for one. And you can read about the Darkrai events here. :) Pretty sure that the one that had a specific location took place on New Moon Island.
In the future, if you have questions about a specific generation of Pokemon, please use the corresponding Q&A thread. In this case, you would use the 4th Gen help thread in 4th Generation Gaming.