I looked up "Saiunkoku Monogatari -Kouriyou no Yume" (黄粱の夢) on the
Japanese wikipedia and this seems to be the 4th volume of the Saiunkoku side stories.
As for volume 7, you can buy it from bk1.jp:
http://www.bk1.jp/product/02594470. Although this is a Japanese website, they do ship internationally.
