Question

What are the major sources of non-meat protein that are found in Seitan, Tofu, and Tempeh?

Answer 1

Those are all materials made from soy beans ... a rich source of
protein.  We generally don't eat soy beans in their original state,
mainly because most people find them yucky, and their taste must be
camouflaged for them to be palatable.

Answer 2

The major sources of non-meat protein that are found in Seitan, Tofu, and Tempeh is soybeans, which is also a rich source of protein.

Answer: Soybeans