Collagen occurs extensively in nature and is derived from natural sources such as skin, connective tissue and bones of animals.
It is the most common fibrous protein found in the organic and physical structure of the vertebrates; that is, birds, reptiles, fish and mammals, including humans.
Collagen is the basic structure of all the skin, tendon, bone, membrane and connective tissue in our bodies and in the bodies of those creatures. Collagen is the structural protein of the animal kingdom and that of humans.

It is to humankind and to animals to what cellulose is to plants.
There are no plant sources of collagen.