You can do a search and find them everywhere.
Pro fit auto interiors has them. http://www.profitautointeriors.com/products/Plymouth/1974/Duster/7076-DUSTERDEMONDART-SPORT---ABS-PLASTIC-HEADLINER---PLAIN/76992/HL70CV-M.html
stockinteriors.com has them, year one.com, and im sure a few others...