If I knew the official name for the tail light bulb access cover, lid, hatch or whatever, I might be able to find one online. My 2009 with tan interior is missing that piece for the right side.
Should be this one, then COVER, REAR COMBINATION LAMP SERVICE, RH #64745-47020-B0 / -E0 (depends on color) (TOYOTA PRIUSNHW20L-AHEEBW - BODY - DECK BOARD DECK TRIM COVER | Japan Parts EU)
It's important to keep an eye on the local wrecking yards... Prius get stripped pretty fast soon as they come in, but best way to make your Prius last is to replace all these little things soon as you notice the need to.
Just got two cans of DupliColor Barcelona red. Did a test on a smooth cardstock box lid with primer, paint and DupliColor clear that I already had. When that all dries fully I'll see how it compares to the car. Top side of the spoiler has almost no paint left. The clear on the tops of the rear door handles is peeling and the paint's discolored. Other than a hood full of rock chips, that's all the paint problems it has. The front bumper cover is oddly perfect, chips are only on the hood. do not know if I'm going to take the handles off or try and fix them on the car. Have to sand and feather the clear out to where it's not peeling then spray on color and clear. do not know why or how clearcoat fail could be just on the rear door handles. If the spoiler goes well I might use several applications of primer and wet sanding most of it off to fill the chips then use the DupliColor on the hood.