@Hitspark If you can (highly recommend this to support the authors), try bookwalker, comic walker, amazon jp, apple/itunes book, honto, there are also ones for more specialized genre like renta, dlsite. As far as I know most of them can only be viewed using dedicated viewer so you will need a bit of extra step to convert the files from viewer format to image formats.
If you want to go the pirate route, there is nyaa, online websites like rawdevart (I would avoid loveheaven (formerly lhscans) because it's riddled with pop-up and over top ads so you can misclick very easily + my antivirus program just goes crazy when I access that site now), or e/exhentai, also forums if you want genres like hentai, yuri, yaoi.