Free Realms Starter
Other than purchasing these Starter Cards, you can also buy Station Cash cards from Target, Blockbuster and other stores (in various amounts). You can even get a free 1 month Membership in the Starter Pack of Topps Free Realms cards.
They've managed to really get word of this product out there and make it easily accessibly for anyone who wants to get a membership. No wonder they have 4 million registered users!
Either way, at only 99 cents I picked up some extra Starter Cards. If you're interested in checking Free Realms out, let me know! The Starter Cards can only be used for new memberships, though... which I learned the hard way.