K.O.
Dollar Tree! They have all sorts of princess items and cowboy hats in different colors. I bought a few of the items for my daughter to play dress up with.
Homemade ones:
Cowgirl-jeans, flannel boys shirt, Dollar tree hat and horse on a stick.
Zombie (if that is something they like)-Dollar Tree has make up crayons. If they are old enough, they can do it themselves LOL.
Movie Star-Fanciest dress they have. Add pearls and costume jewelry. Dollar Tree tiara.
Also, thrift stores have costumes very cheap. They might actually find what they want there.
When you update, let us know what they want to be and I might think of some roundabout ways to get there or a nice alternative.
UPDATE: For the superheros...Bat Girl wouldn't be too difficult. Total black outfit. Might find a black mask cheap at a Dollar Tree or Dollar General or even Walmart. Now, Wonder Woman (who is my fav!) is harder. You could dress her up in red, white and blue for the outfit and use aluminum foil to make the bracelets and tiara. I like some of the others for the youngest's outfit. Hope this helps! OH...just thought of another homemade one! Zoo keeper. Khaki shirt and shorts if you have them. Dollar Tree has little fuzzy monkeys that are adorable! Hair in ponytail if it is long enough. (I ought to know this one. I worked at a zoo for 4 yrs...LOL)