I just commissioned that dress for someone xD
I don't know where you can buy one, except to maybe buy a basic slip dress and alter the bottom hem for the angle. But if you want to make it...
I used a pattern for a bias cut slip dress and just altered the neckline and bottom hem. I used 2 layers of fabric: thin polyester fabric for the lining and chiffon for the top.
For the skirt I used red cotton/poly knit (I think it was 94%cotton/6%poly) and dyed it with purple, black and dark blue dye to get the marroonish colour.
Hope this helps ^^ and good luck!