I found a whole bunch in a place I never thought to look: e-hentai.org Just look for the tag “artbook” and you already find a bunch. Or look for something more specific. Yes, you will have porn in between. Yes, its just the images without text embedded etc. But it’s a decent source.
Basically, the repo for the extensions has been nuked by DMCAs. You can switch to a repo hosted by a third party to bypass it. I’m not gonna link one here because I have not looked deep into who’s a good host. But you should be able to find one with some searching.
Tachiyomi also released a statement here.
They Werke obviously roommates.
If you wanna selfhost your own solution, I can recommend Audiobookshelf. It scraps metadata from Apple, Amazon, Google etc, tracks on a per-user basis at what point in the book you are, can be used in a browser and has an android app. It’s easy to set up with Docker too. You can even add PDFs and epubs to the books.
Been using it for a few months and it has been great.
As for sourcing of audiobooks, aside from torrents there are also audiobb.com & audiobooksbee.com . They use RapidGator as host for a ddl.