Through Gamestop. Go to:
bit.ly/gs-xbox-oneType in your email, it'll ask you to allow the app on your FB account, then the code will show up on the same page. Redeem at
https://account.xbox.com/en-US/PaymentAndBilling/RedeemCodeYea I know, some might not like the idea of allowing an app (you can remove it after) on your FB, or putting in your email. If anyone doesn't mind, and also doesn't need the code, I'm accepting all code donations.
EDIT: Looks like it's over. I guess they were only giving out a certain number of codes, and will do it again next Thursday.