Cinderella is a popular fairy tale known the world over. While many of us think of Disney's idea of the poor-as-a-church-mouse-turned-princess, there are several variations you can use for your own Cinderella costume. In some versions of the tale (with its roots in a fairy tale by the Brothers Grimm), Cinderella wears gold slippers instead of the glass slippers. You can choose to be the classic Disney version or use your own creativity to be a unique version of the princess.
The most well-known of all of the versions of Cinderella is the Disney version. Visit your local Disney store or costume supply store to purchase a Cinderella dress kit complete with optional wig and glass slippers. Disney manufactures these costumes in all sizes at several price points, so this may be your best bet if you're in a hurry.
Go to your local costume rental store and rent an ornate ball gown. It doesn't matter what colour or "period" it is in, as long as it is fancy. Pick silver or gold heels to wear, as the colour of Cinderella's shoes is different in each tale. Purchase a pair of opera-length gloves to go with your dress.
The thrift store will carry a wide variety of dresses from "ball gown" to the "poor version" of Cinderella. Go to your local thrift stores often and sift through the gowns. It doesn't matter if they're in a certain style or time period, as you can interpret her ball gown any way you wish.
Find an old dress or a tan or brown dress (or skirt and blouse) at your local thrift store. It can be a bit loose on you as Cinderella's "poor outfit" may have been ill fitting. Take the clothes and drag them through mud to get them dirty. You can also soak them in tea for about 30 minutes to give them a stained look. Wear black ballet flats or go barefoot. Tie up your hair in a handkerchief and use pencil eye liner to make "dirt smudges" on your face. Carry a broom as a prop.