At Space City Costumes they have a variety of almost outfits. They have robes and tunics that are almost Jedi-ish in nature, a few outfits that are vaguely Trekkish, and these items from their Brown Duster Heroes line. There’s the Brown Duster of course, the Duster Pants, and the aptly named “Crazy Girl Dress.”
These are not the exact duplicates you might get from licensed manufacturers but they’re a hell of a lot cheaper and good enough for cosplay or con wear.