Latemodel Restoration Supply and
Texas Mustang Parts sell carpets, upholstery and some plastic interior parts. Worth a shot.
www.50resto.com
www.texasmustang.com
Other Mustang interior parts suppliers abound; check the Mustang magazine ads and use a search engine to locate more.
Last resort for plastic interior parts are the local boneyards. Interiors are similar on the 4 cylinder Mustangs so you have a good chance of finding what you need there, as far as any hard plastoc parts go.