I would try:
www.brakeparts.co.ukI've had a number of master cyls from them and it seems to work.
You can get into the catalogue as a guest and browse, but only for a limited number of selections ( I think 20 from memory but they warn you if you start to run out)
You need to select "Ford" and then enter "Transit" in the box which will give you a list of all the transit models (well the ones they recognise anyway)
Master Cyls come under "Hydraulic Actuation" and once you have located which master cyl you have, the repair kits (if avail) are listed as "Associated Parts". They list the dia in mm which is sometimes part of the m/cyl casting so avoiding having to dismantle to find out.
Hope this helpful.
Doug