Hi there, Cheryl!
There is absolutely nothing wrong with an adult getting into the Disney Princess spirit and dressing like her favorite Disney royal.
While none of the on-property Disney stores carry those costumes in adult sizes, you might want to look around online for resellers of licensed Disney merchandise. Give the
Disney Store a look, but because Halloween is over, the pickings might be somewhat slim.
Both Toys 'R' Us and Amazon might be good places to begin if the Disney Store doesn't yield any results. Again, retail is heavy on inter holiday items right now, so it might be rough. But never fret; there are all kinds of no-size or one-size-fits-most accessories at Disney that will help you pu;l together the look you are trying to achieve.
Have fun, Your Highness!
See you later, celebrator,
-Deborah B.