Tesla has told me in the past that they do not sell individual lenses. You can only replace the entire taillight assembly and it's not cheap. You might be able to take the lens apart by melting the glue or cutting it open. My guess is that there is a circuit board inside with a driver and LEDs mounted on it. If it's not potted maybe you can replace individual LEDs. If more than one or two have gone bad then I would wonder if there was a leak that let water into the lens, or perhaps the driver circuit has a problem.