The following article contains an answer for the question: What's the dress that the servants buy for Connie meant to be used for? , can you please find it? 

Constance Harding is an unhappy orphan who will soon graduate from Miss Wiggins' school for girls. Her only real relatives are members from the James Clinton family, but they show little interest in the teenager. She is brought to New York by one of their butlers, where she moves in with a bunch of snobs. The upperclass people are not impressed with her, but Connie is able to befriend the servants.
One afternoon, her cousin Barbara Clinton orders Connie to stop Ted Drake from going riding without her. Connie tries the best she can, which results in embarrassing herself. She has secretly fallen in love with him and is filled with joy when she learns the Drake family is organizing a ball. The servants raise money to buy her a fashionable dress. However, Barbara spreads a lie and Connie is eventually prohibited from attending the ball.
Connie is heartbroken, until the servants arrange a limousine she can use until midnight. Meanwhile, the police detain the Clinton family car until almost midnight when they can be brought before a judge, since the chauffeur is missing the vehicle's proof of ownership. At the ball, everyone is impressed with her singing talents. Ted notices her and tries to charm her. They eventually kiss, when Connie realizes it is midnight. She runs off, but accidentally leaves one of her slippers behind. Ted finds the slipper and tries to locate the owner.
Arriving at the ball just before midnight, Barbara spots Connie leaving the ball. Infuriated, she tries to break Connie's confidence and fires all the servants. The next day, Connie is missing as well, and her uncle James berates Grace, Barbara, and Walter for their hostile/indifferent attitude to Connie. Meanwhile, Connie returns to Miss Wiggins' school in the hope of becoming a music teacher. Ted follows her and they reunite in the end.
the ball
In the context of the article, Connie is an unhappy orphan who will soon graduate from Miss Wiggins' school for girls. Her only real relatives are members from the James Clinton family, but they show little interest in her. 
One afternoon, her cousin Barbara Clinton orders Connie to stop Ted Drake from going riding without her and she does so by embarrassing herself. She has secretly fallen in love with him and is filled with joy when she learns his family is organizing a ball. The servants raise money to buy her a fashionable dress that can be used at "the ball". However, Barbara spreads lies about it and ultimately prohibits Connie from attending said event . When everyone else leaves for the ball on the night it's scheduled , however , one servant sneaks out behind them & drives back just before midnight.