There's an excellent source right in Germany:

https://www.herzogleather.com/en/article/gut-leather/ 
Intestinal skin
Cut: strips
Size 1: approx. 20 x 60 cm (10 strips make approx. 1 sqm)
Thickness: extra thin
Size 2: approx. 15 x 60 cm
Strength: thin, medium and strong
Animal species: cattle
Tanning: classic traditional alum tanning

Also known as "fish skin leather" or "zephyr leather".
Suitable for small diaphragms and bellows, etc., in organ
harmonies and mechanical musical instruments. Also used
in watchmaking, especially Black Forest cuckoo clocks.

Musically,
John A Tuttle
Player-Care