Hi,
I have recently put my Mk3 back together after a respray. I had purchased new quarterlight seals but I can't remember where from- probably Paddocks or Rimmers. They come as a length of straight seal, cut over length. They need to be cut to length with an approz 45deg angle on the ends, I recommend cutting oversize and doing a trial fit as it is tricky to get the length right even if you have the original seals. On my car the join was located in the bottom front corner of the aperture, the original seals had a small right angled bit of metal that fitted in the seals at the join- I not sure how much this added to the effectiveness of the seal.