Eeee, ballgowns! My favorite thing.
For the support structure, you're going to want a hoopskirt (and probably a petticoat over it, but a petticoat on its own wouldn't give you quite that much volume). You can find these on eBay, usually marketed for bridal use, but they're actually quite easy to make! I made one using this tutorial
in about three hours for under thirty bucks and I absolutely swear by it.
For fabric, it depends a lot on how you want the finished dress to look. Ballgowns are one of the few things that can look excellent in satin (as long as it's nice satin - you don't have to break the bank, but if it's $2/yd, that's a bad sign) or if you wanted flatter colors and textures you could try cotton sateen (similar weave, different material, not quite as shiny) or gabardine. Basically, print out a picture of the costume and then walk around a fabric store with an eye to texture. Decide what feels right for the way you want to do the costume.
Patterns are where you're going to hit the snag, I'm afraid. There aren't a whole lot of commercial patterns for dresses like this, and if you find one that'll work at all, it might work only for the sleeves, or the bodice, and you'll have to combine them or alter them to get the look right. Wish I could help you more there.