If you've got the old one try East Kent trim they have various sections on a roll.If you haven't got the old one,their catalogue shows the sections they have,you should be able to find something that will do the job.
DOES ANYONE KNOW WHAT PROFILE THE REAR WINDOW RUBBER IS FOR A MK1 TRANSIT BACK WINDOW OF CAB TRIED EAST KENT TRIM AND THERE IS A LOAD OF CHOICES ON PROFILES AND COS WE HAVENT GOT A SAMPLE WE IS STUCK