WELL! I find them just about everywhere, but most of it comes from pixiv.net. It's like DA but for Japan. You can't view anything without an account, though. I believe there's a tutorial out there on how to sign up for it that you can find via Google. From there you search for The Twelve Kingdoms using the Japanese characters: 十二国記 and you'll get quite a bit of results.

Otherwise I find a lot of art by browsing the Japanese fansites. Then following links on these sites to OTHER sites and so forth. It takes a lot of luck too because I don't know Japanese so I just guess on what I'm clicking, haha. But that's how I've been able to found so many.