I have used goldbeater's skin in the past with success.  I had to
recover the intricate primary pneumatics of the Mignon organette.
This used a sort of pigeon-hole mechanism where the output levers
fit loosely into a finely made set of pigeon holes so the pneumatic
cloth needs to be very fine and flexible.  I determined that zephyr
skin would not be thin enough.  Goldbeater's skin is thinner than
zephyr and very strong.  It can be glued with thin hot glue in the
same way as zephyr.

If you need good quality zephyr look no further than Herzog in Germany.
They have three thicknesses and it is of the highest quality:

http://www.lederfachhandel.de/orgelbauleder/leather-for-organbuilders/ 

Nicholas Simons, GB