Well I would first go for a white dress, because both the '95 and the '05 Lizzy wear one in the dance scene. I think empire waist-line would be good. And you should try to pin your hair up like in the movie if you can.
I don't really have any expertise on how to specifically get the costume though. If you can find something in your local costume store, awesome. If not, my next step would be to start googling. You know they have those quick&easy pattern sets to make your own halloween costume, you might be able to find one of those.
You might try going to halloween costume websites and searching things like "1800s" or "regency" rather than just "pride and prejudice". You might even find something from like a Pirates of the Caribbean costume that might work.
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I don't really have any expertise on how to specifically get the costume though. If you can find something in your local costume store, awesome. If not, my next step would be to start googling. You know they have those quick&easy pattern sets to make your own halloween costume, you might be able to find one of those.
Here are some websites you could look into:
http://www.songsmyth.com/patternsgowns..ht
http://www.songsmyth.com/costumerscompaani
http://www.celebrateexpress.com/celebraati
http://www.costumepage.org/tcpinfo2.htmml
www.worldmags.com/magazine/page.ihtml?ppid
You might try going to halloween costume websites and searching things like "1800s" or "regency" rather than just "pride and prejudice". You might even find something from like a Pirates of the Caribbean costume that might work.