I got some Onimusha drawings last year on Yahoo
Japan, but they're not listed very often. Search
'Capcom Yard Sale' with Rinkya or Celga and you
might get a hit. 

If you can find sites with lots of AIC cels, you
may run across some gems. AIC artists do some work
for Namco. I've got a Ace Combat drawing with AIC
studio marks on it. It came stuck to a Tenchi game
cel. :)

Please post again if you find some sites. I'm
still looking for FF7 art. ^_^

KF