A good start is the "Where to find Free CBZ or CBR Comics" page by Robot Comics, the makers of that Droid Comic Viewer app. They list some quality comics. Of course, it all depends on your taste and preference in comics or manga where you go from there. Google is your friend, and a CBZ is just a bunch of images in a zip file with the extension changed from .zip to .cbz (CBR=.rar to .cbr).