I forgot to add that I get the majority of my zephyr skin stock from
August Laukhuff in Germany.  I have been using them years and have
produced 1000s of Wurlitzer valves shipped all over the world with
no problems.  Well, maybe a couple but they were due to neglect.

Laukhuff offers three different thicknesses.  I always use "thin" for
pouches -- great for the 1-inch pouch replacement in the Universal
Piano stack.  "Medium" would be good for pillow pouches, the "thick"
would be good for cuckoo clock bellows, etc.  The "thin" they call
Darmleder Dünn #520560, "medium" is Darmleder Mittel #520567 and the
"thick" is Darmleder Stark #520562.  They are also reasonably priced;
just don't forget to consider the currency exchange rate.

As for Wurlitzer valves, way back when, I had MIDI-fied test valves
and had them energizing around the clock, changing vacuum levels,
reiteration, etc.  Some suppliers didn't make the cut.  Laukhuff's
[zephyr skin] always held up and has the flexibility that is needed
in smaller pouches.

That's the key -- "flexibility".  You can acquire stock from one
company that's .002" thick and .004" from another and depending on
the quality the .004" might have more flex and operate better and the
.002".  Unfortunately there are many differences in zephyr from one
supplier to another due to the way it's manufactured.  I have spent a
lot of time (trial and error) and this would be my best recommendation.
I would also suggest having them cut out the pouches for you.

Here is the direct link to the Laukhuff leather section:

  http://www.laukhuff.de/en/organbuilding/products/leather-felts-and-auxiliary-materials.html 

Download the catalog or give them a call.  The owner speaks English well.

Happy pouching!

Robert Loeffler - Roberts Musical Restorations
Deland, Florida

 [ Darmleder (alum tanned intestinal leather) is found on page 2 of
 [ http://www.laukhuff.de/images/stories/downloads/katalog/5.pdf 
 [ -- Robbie