Where do you get the c gear in black and white?

You get the C-Gear in Pokémon Black and White after defeating the Elite Four and becoming the Champion.

Once you've achieved this, you'll be given the C-Gear by Professor Juniper at Professor Juniper's Lab in Nuvema Town.

The C-Gear is a vital tool for connecting with other players, trading Pokémon, battling, and accessing additional features like the Global Terminal and Dream World.