You're currently accessing that very same website you're talking about. We even have a dedicated section for it:
Manga & Doujinshi
But I know that's not exactly what you meant. So lemme elaborate:
It'd be few to none, since Japan punitively cracks down on copyright infringement, like a rabid dog that chases after any well-known domestic piracy website with extreme prejudice. You'd almost have to do "website hopping" just to be able to consume any sort of pirated content in Japan.
Consuming those H-doujins that were only sold domestically, requires the service (commission) or generosity of scanlators that actually live in Japan. They'll have either to go to Summer and Winter
Comicket (Japan's comic con) if they want to purchase it in person, or use domestic distribution service of shops like Mandarake, Toranoana, Surugaya, Melonbooks, etc.
If said ero-artists are tech-savvy and want to tap into the international market, they'll make an effort to release it digitally too (on multiple platforms). Alas, some JPN mangakas are satisfied with selling physical copies within their domestic market. Remember that Japan is still a relatively insular society.
There are some piracy sites where they offer services similar to what you wanted, but you'll obviously have to pay some "membership fee" (AKA subscription) in order to access their library. If you want free stuff (albeit with somewhat limited library), you're at the right place. Therefore, please be polite and say thanks towards all the generous and dedicated contributors (uploaders, reuploaders, translators, modders, crackers, admins, etc.) that run this site around the clock.
