The Tolino readers are probably the easiest to find, they are sold in a number of bookstores (you can check
here) but I don't know much about them.
The Kobo readers should be available in Mediamarkt/Saturn stores but I don't know if every store stocks them. You need to register the Kobos but it doesn't have to be a "real" Kobo account, any (dummy) e-mail address will do for that.
I'm perfectly happy with my Aura HD, though the one fitting your 6" requirement would be the Glo HD.