You will probably have to wait till winter, but if you dont like waiting you could just set the date to that season on your DS like I usually do.
Giant Chasm
go to the giant chasm and the metangs there learn it
Kyurem can be found and obtained after you beat the main story in the giant Chasm.
Kyurem is found in the Giant Chasm in Pokemon Black and White post-ending. You have to go near the tree in the center of the map.
You can catch them at the Giant Chasm, the open area before Kyurem's cave.
Kyrum
In the giant chasm
You go to giant chasm
in giant chasm
Giant chasm
Giant Chasm
giant chasm
it is at the end of giant chasm
After you beat the elite four you can catch it in the giant chasm there is also a legendary Pokemon in the giant chasm called kyurem.
in the giant chasm
In the Giant Chasm when you get up to that part
At the giant chasm