If you're not opposed to it, there's a good tradition of short form horror manga

Junji Ito has several collections of his work outside of the big names (Tomie, Gyo, Uzumaki), just look up Museum of Terror, it has at least a dozen volumes or so of stories (my personal favorite is The Thing that Washed Ashore)

Outside of Ito, there's a lot of other stuff, but I think a good place to start is an older book known as Fuan no Tane (translates to Seeds of Anxiety)

It's one you're going to have to find translations of online, but it's got some genuine creepy stuff that ranges the gamut from more traditional ghost stories to just out and out disturbing nightmares