The absolute best is made by Ethicon from the intestines of Persian cats bred from birth and lovingly cared for in Ethicon's facility in San Angelo,Texas.
At the appropriate age (9-12) months, the cats are strangled by Jose Ramirez, a 20 year Ethicon employee and disemboweled immediately. The intestines are cleaned, preserved, air-dried and sized.
Needles are attached, the sutures packed and sterilized and shipped all over the world.
catgut (kàt´gùt´) noun
A tough, thin cord made from the treated and stretched intestines of certain animals, especially sheep, and used for stringing musical instruments and tennis rackets and for surgical ligatures.
sheepskin (shêp´skîn´) noun
1. The skin of a sheep either tanned with the fleece left on or in the form of leather or parchment. 2. Informal. A diploma.
And just to add to it, in North America elkhide is the preferred material for bagpipe bags... except... bagpipe elkhide comes from cows, not elk. Oh well.