Inara's Navy Dress most famously appears in Trash, with Inara holding a gun on a trash-covered Saffron. It also appears in Out of Gas (in a flashback).
More pictures after the break...
Some screencaps from Trash:
Screencaps from Out of Gas:
Shawna put up some of her costumes for auction a few years ago, and this was one of them:
Thanks to Moara for pointing out this Behind the Scenes pic:
My Photobucket Album for Inara.
Serenifly Costumes site for Inara.