FiftiesWeb has some great info on fifties fashion, they even show pictures. Here is the link to the fifties fashion page on their site: http://www.fiftiesweb.com/fashion/fashion-w.htm
Fashion-Era also has great examples of formal wear from the fifties. They show photos of both super fancy gowns and more casual cocktail dresses. This site provides a detailed list of what women in the fifties wore at each time throughout the day - home, running errands, dinner parties, fancy parties, work, etc. Here is the link for their fifties site: http://www.fashion-era.com/Vintage_fashion/a10_collecting_fifties_vintage_1.htm
Remember, many women tailored clothing to their own particular style because they could not afford to buy fancy, designer clothing. So you might try hunting for some fun, vintage pieces at thrift stores and vintage clothing stores and then mixing them, adding accessories or changing little details to make it more of the fifties style your going for. eBay is flooded with cool vintage accessories. Look for a seller with a lot of positive feedback, there are lots of dresses, earrings, and all kinds of vintage items there. One more idea- swing dancing is extremely popular right now. So a lot of women who have gone or go swing dancing have some kind of hoop dress or other outfit with a fifties vibe. Swing dancing websites also show pictures of guys and girls in typical fifties party clothes.
Poodle skirts are some of the most widely used outfits for people going to costume parties, 50's themed events, etc. They are easy to find these days at costume stores and vintage websites. There are some really cute silky poodle skirts that look more dressy than just the standard material. A petticoat over the dress was also hugely popular in the fifties. You might also want to see if a grandmother, a friend or another relative has some vintage clothing or accessories that you can use.
If you want to purchase something in person, a costume store, vintage store or thrift store would be your best bet. If you want to look online you can go to froogle.com (Google's shopping website) and type in Fifties Dress.
Let me know if this answered your question, if not, I will do more specific research for you. Thank you!