There are many places online to shop for Kindle 4 cases and covers. Although it seems logical that Amazon would have the best covers and cases for their own device, we have found that this is not always the case (no pun intended).
Amazon does have a large selection of Kindle 4 covers, but it is hard to sort through them, and find ones that are truly unique. That being said, there may not a store out there that rivals that sheer size of the selection.
Ebay might be the only competitor when it comes to huge selection on Kindle 4 covers, and there are some really amazing ones on there. It is a little easier to browse the selection on Ebay, but you have the trade off of not getting new cover, but instead one that is used and may be in good condition.
There are a few websites out there that sell special covers, either unique in material or in design.