Nordstroms.com has a wide selection in styles and sizes.
Silhouestts.com has a great selection of all types of cruise closthes in sizes 14W-38W.
Borrow something from a friend or check out the vintage dress up clothes in your grandmothers closet. Grandma's is also a great source for evening bags and glitery jewlery.
Buy some new jewlery (there's a lot on sail after prom season) to dress up something you already own.
One thing about dressing for a cruise, you don't have to buy new clothes, 99% of the people have never seen you wearing that outfit before.
If you're cruising the Caribbean or Mexico, you'll find lots of resort wear that is suitable for the 2nd formal night. It will also give you a chance to show off your new tan.