Pokémon Global Link website, absolutely free. Each one of the skins will be available for a limited time only, so make sure you grab them while you can!
Each C-Gear skin has its own unique password; once you have that all you need to do is log in to the Pokémon Global Link website* and use the password in the promotions page. This will unlock the skin for you to apply in-game whenever you fancy.
The Charizard C-Skin is available between October 13th 2011 and February 16th 2012 with the following password: EUPGL102011