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).
Disguise Maleficent Disney Adult Costume Black Deluxe Movie Women Gown halloween
Time Remaining: 29d 16h 45m
Buy It Now for only: $35.15
Maleficent Adult Women Dress Fancy Costume Halloween Cosplay Black Medium Size
Time Remaining: 22d 16h 12m
Buy It Now for only: $32.38
Womens Adult Disney Wicked Wear Deluxe Maleficent Evil Queen Costume
Time Remaining: 6d 2m
Buy It Now for only: $44.95
Maleficent Aurora Coronation Gown Adult Costume
Time Remaining: 22d 23h 10m
Buy It Now for only: $19.99
New Disney Maleficent Movie Character Black Horns Adult Womens Halloween Costume
Time Remaining: 18d 17h 44m
Buy It Now for only: $20.23
Brand New Disney Vile Villains Maleficent Sassy Adult Hallowee Costume
Time Remaining: 9d 22h 28m
Buy It Now for only: $26.99
Womens Disney Maleficent Disguise Deluxe Adult Black Horns Halloween Costume
Time Remaining: 28d 13h 8m
Buy It Now for only: $27.46
Elite Deluxe Sexy Dark Sorceress Adult Womens Costume NEW Maleficent
Time Remaining: 18d 2h 53m
Buy It Now for only: $87.58
Women Adult Disney Movie Costume Maleficent Christening Black Gown Party Outfit
Time Remaining: 5d 10h 56m
Buy It Now for only: $35.18
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!