This site has cases/covers for the PRS-300 and 600, plus just about any other ebook reader you can think of. I found it during a google search. It sort of looks a little big for the 300 defeating the purpose of a reader I can carry around in my pocket. But I'll probably get one for traveling.
http://www.extremeinkjet.com/Electro...rmor_s/655.htm
Regards, Daryl