Cannot help you with official covers other than suggest you look at eBay via that link in my post above, or see if some of the online stores have old stock.
http://en.wikipedia.org/wiki/Sony_Reader
This link just above has the sizes of each ereader that provides a ready reckoner to assist in getting a "substitute" but I doubt an official Sony ereader cover for a specific model will fit another model well.
Another option would be to check the Mobileread marketplace.