I've got cases for everything and they're normally kept in them unless I'm using a particular one at home, then I don't bother because of the extra weight any case adds; that, for some reason, annoys me when I'm reading.
They are all in different brands and styles, depending on what was on-sale and rated well when I was in the buying mode. Some came with covers as part of a package deal when I bought the device - the Nook HD came with the B&N Nook HD cover, and the Fire HDX came with a Belkin Chambray cover. I just bought this one for my PW2:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
It was a dollar cheaper when I bought it.