A recommended source for all sorts of leather (including intestinal
leather or, as you call it, Zephyr Skin) is the German firm of
"Leder Herzog"  where I buy all sorts of leather. They ship
worldwide and sell high quality products only. Visit
http://www.herzogleather.com (English website available) 

I know a friend from The Netherlands who was restoring a Mignon
reed organ from Leipzig and asked me how to obtain this sort of
skin. He was satisfied about the offer by Herzog. Maybe you may
give a short feedback to that source if you're going to buy there.

Regards,
Uwe Gernert