Is the seal actually damaged? If you can't see a tear or a hole it may be a case of setting it back into it's correct position as it may have just come away slightly from it's mounting points.
If it is damaged and you have been unable to find a replacement online, you could try removing the seal, clean and totally dry it and then repair the damage from the back face with a puncture repair kit for tyre inner tubes.
Rupert, January 2020