All the costumes in the movie were designed by Ann Roth, who is an Oscar winner for best costume design (and after seeing all the costumes in that movie, i can see why! they’re gorgeous!) The dresses are one of a kind though, so you can’t buy them anywhere.
If you were needing something similar, i would probably hire someone to design them for you, although that’s expensive..

i tried to look around for some that looked similar, but couldn’t find any..sorry:(