I find that some "modest" dresses work well for formal events like weddings, etc. If you Google something like "Mormon bridesmaid dresses" you might find some that could work well. That's what I did for my wedding...I could not find a wedding dress that worked....I ended up with a gorgeous dress from a Mormon wedding dress store online.
Here's an example...good luck!
If you look at something like latterdaybride dot com and look at their bridesmaid dresses, they have some cute ones!
ETA: Just looking at that site's prom dresses...they have some cute long formal dresses on there too.
