Are there any sources for high resolution (digital) manga?
Hate to write a suggestion in an answer, but too new to comment.

Have you tried crunchyroll? If you pay for their premium membership you're supposed to get HD anime and manga (you can get a manga only subscription).

If you're looking at fan scanlations, some online reader sites downgrade the quality of uploaded chapters in order to make the pages load faster, which will result in jpg artifacting at times as well. If you're looking for higher quality, I suggest grabbing it from the scanlators' sites and/or using their readers if possible.