Image have their own store that allows DRM free downloads in a variety of formats.
Comixology also allow DRM free downloads in PDF & CBZ for
certain publishers. Check for the download icon next to the comic in question. Most of the participating publishers have all their comics available for download but a couple (Dynamite, I'm looking at
you) only have some of them DRM free. There's a list of them
here
Have fun, I've spent far too much on these