Yes Scarlett O'Hara the fictional charector who lived in the American Civil War.
What does she have to do with a costume party set in the 18th century/Regency?
She made a dress from curtains! And you can too if you choose the right curtains.
For regency you want to keep your eyes open for sheer cotton curtains, maybe with light embroidery on them like these from Target?
When it comes to the 18th century, you may want a quilted petticoat, keep your eyes open for a quilted coverlet of silk or cotton. Solid colors only.
You do want to stick with the natural fibers of Linen, silk, cotton and wool. you will likely find that the embroidery is done in a synthetic thread.
You can sometimes save money this way, though sometimes you may not. I would suggest you price the yardage first, then look at the premade curtains.