To my complete shock and amazement, Grave of the Fireflies is, once again, out-of-print in the US. The film was held by Central Park Media for many years until they went backrupt a few years ago. Then ADV bought their catalog and reissued Fireflies with the same content and a new cover design. Then they, too, went out of business in the latter half of 2009.
If you dig around, you may find a CPM or ADV copy of Fireflies. Personally, I'd rather hold my own with the most recent fansub copy, or import the Ghibli ga Ippai DVD from Japan. I've noticed that the picture quality on the Japanese disc is much better, more honest on the colors. CPM made some "remastering" adjustments to the picture for their Special Edition release some years ago; I'll leave it to you to decide which version works best.
I'm holding out high hopes Ghibli will bring Fireflies to Blu-Ray in 2011. If you'll notice, you'll see that the Miyazaki movies are being released in chronological order - does this mean My Neighbor Totoro is next? If so, I think it's only fitting that Takahata's searing war drama appear alongside, like it's 1988 all over again.
I am still amazed that Grave of the Fireflies and My Neighbor Totoro appeared on a double bill in Japanese theaters. And I remain astonished that a single animation studio could create two masterpieces simultaneously.