So, this is the British military, but it's still really cute! MADE TO ORDER British Military corset and puff by LyndseyBoutique. OK, it's Dieselpunk rather than Steampunk, but I like it anyway and would totally rock it at a steampunk party.
Antique Lilac sells dresses, as well as offers tutorials on how to make outfits. An excellent site with a beautiful gallery for design ideas. I know it's a doll, but what an amazing fairy costume this would be!