Finding a plus size prom dress is not easy. Most stores max out at size 12 or 16. Thankfully, the internet is full of great buys for the curvy girl. Many of the shops I've listed below have hundreds more options. If you're worried about the fit, read the return policy carefully, and check if what you don't like can be fixed with tailoring.
dot mermaid hem dress, Sydney's Closet $149
red swing dress, Pin-Up Girl Clothing $110
iridescent pickup dress, Sydney's Closet $199
lace covered dress, Carraz Gowns $293
coco feather trim gown, Carraz Gowns $425
pink obi dress, Sydney's Closet $299
gold Marylin Monroe dress, Prom Dress Shop $289
black floral applique ballgown, Sydney's Closet $199
burgundy applique dress, Prom Girl $99
Bridal shops and ebay are other great places to find plus size formal dresses.
Please note, eDressMe carries some great plus size prom dresses, but I recommend finding them elsewhere. If you're plus size, that store wants to charge you a ridiculous $25 surcharge.