Sources of Complete Protein. Animal sources of protein are complete proteins, and some plant foods contain all essential amino acids. Sources of complete animal proteins include milk, yogurt, cheese, meat, poultry, fish, seafood and eggs. Soy protein is a plant-based source of high-quality, complete protein.ncomplete proteins are found in most grains, nuts, seeds, nut butters, green peas and legumes (such as lentils, chick peas, black beans, pinto beans and navy beans). Pairing Incomplete Proteins. You don't have to eat complete proteins to get the essential amino acids your body needs.