If you want to make a dress yourself, you can use or buy adult size T-shirt in black or purple and you can combine both of them or sew your own little dress for kids.
Image by PublicDomainPictures, Public Domain License, Pixabay
It actually depends how old is the girl that you are making this costume for. If she is still a little one, T-shirt will be enough, and if she is taller, you can use two T-shirts or additional fabric. If you are going to sew the dress from scratch, make sure that you'll buy enough fabric.
You can add a belt in black, purple or orange and it can be just a piece of fabric or even a scarf.
You can also get very simple black hat (they are really inexpensive) and add a buckle. It can be just cut out of colored paper and you can simply glue it to the hat. And you can even add few orange and purple little (paper) stars and make an ordinary hat very attractive.
And the last thing you'll need is a broom. If you have a little broom at home, you can use that one. Or you can easily make one out of cardboard or/and fabric. Another option is to buy one inexpensive broom and add few details to make it unique. You can use pieces of black, purple, orange and green fabric and wrap it around the broom.
And don't forget to put on some make-up.