What a sweet, romantic guy! Your fiancee is a lucky girl. I did a little research for you. It seems the dress was created by designer Monique Lhuillier. You might try contacting a store (contact info here: www.moniquelhuillier.com/index2.html)
Another option you might try is taking a picture of it into wedding and prom dress stores. They might be able to point you in the right direction. God bless and best wishes to you both!
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