Elegant dress to wear to a wedding?

I found some dresses I think fit what you're looking for at DressProm.net. I know their name has Prom right in it, but they sell a ton of dresses, and a lot of them are more cocktail dress than prom dress :) I've included links below to ones I came across that looked the most in line with your style and are available in an 18. Hope you find something you love!