Where can I find a keldeo in Pokemon black
You can only get it from a gamestop event that has already passed so now the only way to get it is to trade either from a friend, online, or GTS. Otherwise, you are out of luck. Source: http://bulbapedia.bulbagarden.net/wiki/Keldeo
You can only get a Keldeo from the event(already passed) or by trading it through GTS or with a friend. Also you can get it with AR or hacks.
You can't find keldeo, since it is only an event Pokemon. The event though expired a long time ago.
