There are a number of RPs that offer webbies as rewards. I like MyPoints and CreationsRewards but I also get webbies from PCHPoints, LuckyPoints, Memolink and my Chase-i credit card.
Sign up for any or all of those programs and earn your points there. When you have enough points to cashout, request your webbie. After you'vre received your webbie via email, go to webcertificates.com to redeem the code. If this is your first webbie, you will have to set up an account with them in order to redeem the webbie. After that, all subsequent webbies can be added to that same account.