I did a quick internet search and found this kit from Ecklers.
GT/CS Stripe Kit; Black(1968 Mustang) $164.99
Item EK-44-907010
Precise Restoration Kit. The GT California Special Stripe Kit offers an authentic exterior enhancement designed to replicate the original factory appearance. It
www.ecklers.com
Sounds like what you are looking for.
You can do an online chat or call them. Phone number listed on the link
Application. Specifically designed to fit the 1968 Mustang GT California Special, this stripe kit aligns precisely with the
vehicle’s original styling and contours, maintaining authenticity.
Precise Restoration Kit. The GT California Special Stripe Kit offers an authentic exterior enhancement designed to replicate the original factory appearance. It is ideal for restoring the distinctive GT stripes on muscle cars, delivering a showroom-quality finish that meets the highest standards.
Durable Material and Finish. Made to meet or exceed factory specifications, this kit includes high-quality decals with accurate color matching and texture. Each kit is supplied with a squeegee and clear application instructions to help ensure a professional result and long-lasting durability.
Effortless Installation. The application process is simple and requires no drilling or permanent modifications to the vehicle's surface. Designed for ease of use, it is suitable for both experienced restorers and enthusiasts performing self-installation.