I was able to get a Lockheed 5 1.2" Servo kit out of
http://www.jlspares.com/ in the UK.
They can sell you the whole kit, and they can also sell you the kit minus the large diaphragm. This second option reduces the cost by better than half from memory.
Neither of the kits include a replacement air valve diaphragm. These air valves are a curious item as they are engineered without the benefit of an easily replaceable diaphragm, so as you take yours apart be very careful to not damage this part.
They can also provide a replacement push rod if the hard chrome has come away from yours.
Quote:
Type 5 1/2
Exchange rebuilt air valve £40.00
Hydraulic seal kit and gaskets £22.50
Exchange Vacuum piston push rod £20.00
Vacuum chamber diaphragms £25.00