Where do you want to buy it from? If it's some obscure shop that has its own encryption scheme, it may be hard to find any solutions. If it uses Adobe encryption, you can probably get away with using the DeDRM tools. Whether it's a PDF or EPUB, you should be able to extract the images. EPUB is easiest, since it's just a zip file, just like cbz. I've had de-DRMed EPUB files that I opened directly in a comic reader and they worked perfectly fine. The image file names were already sequential.
If it's some other scheme that hasn't been cracked yet, though, and you don't care about having the original files, I've had success using this method for automating screenshots. I think this is a great option if you just need "good enough" image quality to store something without DRM, as it will work on pretty much anything. And tbh, the image quality of digital comics isn't that great anyway.