If you can get darkrai outside of mystery gift in a legit way I'd like to know where, when, and how.
I'm sorry to say,but no. The only way to get that nowadays is Pokegen or AR. You might be able to get it through trade, but someone might trade you a hacked Darkrai so watch out. Also it might be hard to find one.
No, it's not possible. But now there's a event live on WiFi for Darkrai in Black and White.
You can also trade
Hope This Helped :D