The
Digital Comic Museum has golden-age, public-domain comics in cbr/cbz format (they'd need to be converted), and Underground (an indie comic) is
available in CBZ or PDF form.
Standard-size comics don't do well on 6" ebook readers, even using the landscape view. Some manga, with smaller page sizes, would do better (I don't know any sources for free manga), but it's still more picture than fits well in that screen size.