Definitely Moneybrokers, it's the only way. You have to go to
and set up an account first. Oh and when you register your card make sure you say it's not going to be used for online gaming/gambling. Your credit card company will not accept it.
Of course after all that's done then you just sign in on the webzen page and press wcoin at the top and buy w-coin, then select credit card etc, etc.