Manga publishers

If by manga, you mean Original English Language (OEL) manga, there used to be a few like Tokyopop, Del ray, Yenpress. But nowadays, one is more or less defunct (dropped manga last year), and the other two do not deal with OEL anymore, only their in-house projects like GN adaptation of movies/novels and buying foreign licensing rights.

Seven Seas might be close to what you're looking for:

Seven Seas is always looking for talented manga artists for future projects, so if you are an artist 18 years of age or older, please feel free to write to us with a link to your online portfolio so that we can see your high quality, sequential art. Please note that we are currently not accepting any form of unsolicited story proposals and/or submissions from writers. We will also not supply any form of art critique or advice on how to become or "make it" as a manga artist, so please don't ask.

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But if you're writer looking to shop your original manga. My advice is to treat it like another comic project, not specifically Manga, and shop it to publishers that deal with original GN, like Oni, Images, SLG, Top Shelf, etc. Warning: no money upfront.