I love this dress that Sienna wore on the set (note: this isn't actually from the movie, they didn't have cellphones then. It's just Sienna wearing one of the costumes). It's interesting and fun but still quite elegant. I did a bit of hunting around for a similar dress and the closest one that I could find is the dress below - $58 from Urban Outfitters. The boots are Calvin Klein, currently on sale for $149 (down from $200) from Macy's.
The outfit is very simple, but can easily be accessorised to match your style. You can also replace the boots with ankle boots or boots without heels for a more relaxed look. This is one of those outfits where less is more.