Try Valiant Miniatures they have many different oni and other odds and ends from their legend of the five rings line of minis, I actually have some and they are pretty good representatives of the ogres. Kensei also has a verity of Japanese demons and monsters. Personally I would love to see (and buy) some of the Youki in plastic.
The Japanese did use artillery but tended to use incendiary or fire-starting ammo and were almost exclusively used in sieges. They had three types of artillery that I know of: trebuchet style catapults, cannons that were used as semi long rang flame throwers, and fire arrow lunching mortars called a Bo-hiya that could be carried by one man. In my opinion the last one would make a really cool set.
I read about the catapults and cannons in books, but I found the Bo-hiya here: