Nuts, seeds, yogurt and other dairy products, non-dairy products [soymilk, almond milk], poultry [vegetarians do not eat meat or poultry though].
Quinoa has all essential amino acids and protein, as well as buckwheat I think and several other grains.
Fruits and vegetables also have protein.
Basically every food you eat has protein in it.
Me - common sense - cookbooks - research.