it's confirmed on celebritystyleguide.com that emma's dress is an ossie clark.
http://celebritystyleguide.com/?op=item&sid=22&cid=303&page=1&item=8652&rp=2
She knows already. [Try to read the original post/replies beforehand]. She wants to find a similar dress.
maybe forever 21 has something like this? i've seen maxi dresses kind of similar to this in there.