If you cannot get one you can still strip it out and bed it in black silicone/mastic to effect a repair. This means that the roof still works and is leak free. It is not a hard job but it is difficult to avoid scratching the upper surface of the hatch.
I did this about 2 years ago on my Mk2 and it is still fine today. If I remember correctly, the Haynes manual is not bad for this job for a change.
Good luck!