You are correct, it was a custom made piece for the show.
Actually, I have already made a muslin for this dress based off an 80s prom dress pattern (with modifications: built in bra since Ms. Paquin is a small B I would say and I am a large D, also I legnthened it a little to just past my knees instead of just above) If it works out, I will be all over posting a tutorial
My only key piece of advice is either interface the bodice with some serious interfacing, or sew featherweight boning into the lining. Unless you have washboad abs or a wardrobe team on hand to adjust your dress, a low waist dress is going to bunch in bad ways after a half an hour of wear.
The key is the fabric, I have had very little luck finding a suitable fabric. It would seem they are trying to evoke the red-work quilting that was popular during and after the Civil War (or my nerd tastic mind just went there on it's own) However most red work is smaller than the pattern on Sookie's dress. Also, her skirt is pretty full (you can see when Bill whisks Sookie up at the end of the episode) so finding a fabric with lots of movement without looking like...well, an 80s prom dress...is a complex endeavor.