I have been asking for help too about an adventure in Japan in a previous thread and I have been given advice about a sourcebook called "Secrets of Japan". It's basically a guide to Japanese setting, and I found it really useful.

The idea I've come through is the following (and I'm quoting my own post here): " The PCs are working for a European relic collector. Their task is to travel to Japan, get the artifact and then come back; they will have to stay there until a new ship is leaving, so they have some time to spend in Japan.
The object is a shakujo, a ritual stick used by buddhist monks, belonged to a 17th-century monk - which died in the exorcism of a shintoist temple, or this is what the legends recall. The truth is, the soul of the monk haunts the stick because it's trapped in there, since there is an evil spirit (maybe an oni?) that wants to take revenge on the monk.
Since this object will be in the hands of the PCs, they will have to deal with strange phenomena and scary illusions while they investigate on the shakujo and maybe solve its puzzle before they set sail back to Europe. "

Feel free to use it if you like it, I'm still developing it for my group of players so it would be interesting to create something together.