I don't know how proprietary the seal is and general availability of new replacements, but the exact seal appears to be a discontinued part.
You may be able to get a used one from forum member Robert Foster. He is currently refitting his Westy and potentially enlarging the upper windows this spring. However, he stated that "any salvageable weather seals" will be among the components for sale with the windows. There are currently two forum members in line for the windows. If they don't end up buying the windows or wanting the seals, it may be a potential source for you. Hope this helps.
Mark, If you haven’t solved this yet, I replaced my upper window seals with a standard product. I measured the old seals, and the flange size, and matched this to something similar to the Trim-Loc seal. I was able to easily replace the seals on both sides. There is now a seam where the two ends of the seal meet. I put the first one at the bottom, and then when I looked at it, it looked better if the seam was at the top. One of each. That was over a year ago & no leaks. If it’ll help you, I may be able to find the exact material I purchased. Michael/ Tortuga44.
I assumed this request was about the upper sunroof window (Heki). If it is for the two upper side windows, I used this and worked well. Maybe the same product that Tortuga44 used. Will have lots leftover .