I use to get zephyr from Organ Supply Inc. (OSI) but I know Laukhuff
in Germany is also selling zephir in three thicknesses.  In section
5 of their catalog, under the description "gut skin", catalog numbers
5 205 60 - 5 205 61 - 5 205 62.

But it might be a good idea to call them and inquire about the product
and the most suitable thickness.  Their website: http://www.laukhuff.de/ 

Good luck!

Gilles Chouinard, organbuilder
Laval, Quebec, Canada