Does anyone know who designed the dress that BONES（ボーンズ）-骨は語る- wore in the episode A boy in a bush? Its the dress she was wearing to go to the benefit.

Check the credits at the end of the episode… there’s often a mention of special wardrobe designers if a character wears something outside if the norm.
For instance, I seem to recall there was a wardrobe credit at the end of Bones/Booth’s wedding episode.

Keep in mind that if it was just something whipped up in house によって the wardrobe dep’t, then there’d be no special mention.