Generally, I think people are finding the Smart Cover in Apple stores, or Best Buy or Walmart or Target. Seems pretty easy to find in stock, although some colors may be more elusive than others.
To protect the back, if that's important to you, consider a Best Skins Ever skin (don't get the screen protector tho) or look into a Marware shell or upcoming Incipio Feather that's supposed to be smart cover compatible.