Cinderella's wicked stepmother is a Disney costume that is quite rare; in fact, I've only seen parts of it. Not quite a whole costume, but here are some ideas on putting one together.
So what could you do, to put together such a costume, if you wanted to play Cinderella's wicked stepmother?
For one, the story says she is very vain. That being the case, I would expect plenty of makeup and a fine dress. Not to mention plenty of jewelry to go with it!
As for the hair...hmmm. I'd either choose an upswept brunette or gray wig. Maybe put some more jewelry on the wig (like a fancy comb). I go back and forth on the hair color. If she's vain, she might try to dye it. On the other hand, dyes weren't all that common back then. So, your choice on how to handle it!
The problem with this costume is that most people won't recognize it. Unless, of course, you are going to a Cinderella party!
OK, so you don't want to go as Cinderella or the Fairy Godmother. If you're still interested in playing, umm, a "bad character", can I suggest Maleificent from Sleeping Beauty instead? This costume is fairly easy to find in adult sizes (below).
WOMENS DISNEY MALEFICENT DISGUISE DELUXE ADULT BLACK HORNS PARTY VINYL COSTUME
Time Remaining: 10d
Buy It Now for only: $20.76
Womens Disney Maleficent Disguise Deluxe Adult Black Horns Halloween Costume
Time Remaining: 5d 23h 44m
Buy It Now for only: $23.85
Maleficent Costume Deluxe Adult Disney Halloween Fancy Dress
Time Remaining: 15d 7h 57m
Buy It Now for only: $57.69
Disney Villain Sleeping Beauty Maleficent Adult Womens Costume Dress Large
Time Remaining: 19d 9h 31m
Buy It Now for only: $39.95
Disguise Womens Disney Maleficent Movie Costume Staff Multi Adult
Time Remaining: 25d 23h 31m
Buy It Now for only: $23.41
Adult Maleficent Wings
Time Remaining: 27d 9h 17m
Buy It Now for only: $11.65
Maleficent Movie Disney Deluxe Maleficent Adult Costume Horns
Time Remaining: 25d 8h 1m
Buy It Now for only: $25.98
Disneys 2014 Movie Maleficent Christening Black Gown Deluxe Adult Costume 71825
Time Remaining: 19d 8h 2m
Buy It Now for only: $39.98
Disguise Womens Disney Maleficent Movie Maleficent Deluxe Adult Horns Costume Ac
Time Remaining: 24d 19h 44m
Buy It Now for only: $22.94
While I haven't been able to show you a Cinderella's Wicked Stepmother costume, I hope I've given you some ideas. Enjoy the hunt!