I can't find one. What I can suggest is the Poetic case that I bought. The front of the case is like the smart cover, complete with the magnets that wake the iPad and put it to sleep, and the back is a clear plastic.
I understand, just that the Poetic Case is currently under $10, so it isn't going to cost you anymore than a back cover. It gives you a case that will protect the back, and then you can either return the Smart Cover (if you like the Poetic case enough), or trade off between the Smart cover or the Poetic depending on if you want the back protected. It could also be a temporary solution until back covers become available, and then become your backup.