you need to trade from your own victini from your own B/W from getting the liberty pass and going to liberty island. This requires a past event that is over so you cant get it. There was also a second event for a victini with fusion flare, fusion bolt, searing shot, and v-create but that is already past, so you are kind of out of luck. There was a third event that just ended in japan where you get a victini with glaciate, bolt strike, blue flare, and v-create so if you live in japan you cant get it anymore. But this means that there may be a pretty high chance that it will come everywhere else so if you live somewhere else just wait a bit because there is a high chance that there will be another victini event. You could always ask someone for a trade or go to the GTS if you seriously want it.
So there is no one that trades you a victini. You have to get it from an event from B/W that already passed. So now the only way is to wait and hope or trade someone.(Just to clarify because there seems to have been some discrepency)
Source: http://bulbapedia.bulbagarden.net/wiki/Victini
