The costume comes with the dress and necklace, although if you look at the images from the movie, you can see that the necklace Blair wears with the dress is more of a Sailor Moon-type heart necklace with a crown.
Claire's has a pretty good necklace that can pass for almost an exact copy of the necklace Blair wears in the movie.
Floral Heart Shaped Locket Pendant Necklace
originally 6.50, now 5.00
The costume does not have a crown, luckily Claire's also has a crown that is similar to the one Blair is wearing.
Unfortunately, it appears to be sold out. However, Claire's and party stores such as Party City probably have similar crowns.
Here's hoping there are more Barbie movie costumes for adults soon!