You can use the Neko fork of Tachiyomi to import the library of your Mangadex account. After that you can make a backup in Neko and then take that backup and restore it in regular Tachiyomi or any other fork you want to use.
For any other site i think you'll just have to do it manually.
you can read the official translation on nyaa.si - go Literature --> English translated.
you can also read it from mangasee123.com but the quality isn't high
Category>Literature>English translated
I've got an Ipad now so that I can work through some of the classics like Naruto, Monster, Ruoni Kenshin, Akira, etc :)
yeah there's often big collections of light novels uploaded. here's a link
https://nyaa.si/?f=0&c=3_1&q=light+novels&s=seeders&o=desc
They're talking about a file type that can be read by apps you use for, well, reading.
If you're looking for those files I'd go onto GetComics.com or go to Nyaa.si and make sure the files in the torrent are explicitly .epub.
I would recommend searching on tachiomi , as more people suggested.
If it is something like an old and rare stuff. Try searching in r/OpenDirectories. It's wiki has an guide how to get started.
If you still can't find any sources. All you can do is to wait for it to be released or buy the stuff and scan and upload.
Amazon has it for cheap.
Havent been able to find a rip of it yet, so i just bought it on kindle.