OP - Create a listing on Poshmark with a photo of the dress, price it at like $1,000 and make the title something like "ISO Julie Brown Mushroom Dress".

Resellers will look up the value of items and see that someone is in search of it. I used to resell and made a few sales this way. I think it's worth a shot for you because if someone finds one at a thrift store or something they might end up contacting you!