Usually CC-related youtubers get a bunch of secret codes from IGG that they give away to everyone who fulfills their criteria (subscription etc.). I've seen Distaggio and DylanDoesGamesAndStuff do this. I'm not familiar with any other way and haven't seen any codes on social networks either.
Today the official link account posted some codes in the morning. They were all already used less than an hour later, so chances to get some are thin, since those can only be claimed ONCE. And therefore the first five who claims the codes get the rewards, for the rest those will be useless.