Simple answer, the same places many animals get THEIR protein – from plants! Do cows eat other cows, or chickens, or pigs? Of course not, they eat grass! Many of the largest, strongest animals in the world never eat the flesh of other animals: elephants, hippos, horses, oxen, gorillas... For those who might rightfully point out some animals ARE carnivores, I would answer that I accept as truth, the Biblical mandate given at creation: “Also, to every beast of the earth, to every bird of the air, and to everything that creeps on the earth, in which there is life, I have given every green herb for food”; and it was so. Genesis 1:30
Thus, in the newly created, world animals lived peaceably. It wasn’t until after sin and death entered the world that animals began to eat other animals! However, the Bible teaches that in the earth made new, this peace will be restored! "The wolf will live with the lamb, the leopard will lie down with the goat, the calf and the lion and the yearlings together; and a little child will lead them. The cow will feed with the bear, their young will lie down together, and the lion will eat straw like the ox.... They will neither harm nor destroy on all my holy mountain." Isaiah 11:6-9.
Theological discussions aside, where DO vegans get their protein? From a variety of legumes (dried peas and beans), nuts, seeds, and whole grains.
I found this summation which concisely answers the introductory questions on the website Vegetarian Resource Group (http://www.ivu.org/faq/protein.html). Please note that this brief article puts to rest the notion that obtaining sufficient "complete" protein at each meal required combining a variety of foods high in specific amino acids. A theory, since proven untrue, that made planning vegetarian meals seem like a complicated task, requiring extensive knowledge.
Official recommendations suggest that eating 8% of our daily energy as protein will provide an adequate amount. (These) ... recommendations... are based on animal sources of protein such as meat, cow's milk and eggs. Plant proteins may be less digestible because of intrinsic differences in the nature of the protein and the presence of other factors such as fibre, which may reduce protein digestibility by as much as 10%. Nevertheless, dietary studies show the adequacy of plant foods, as sole sources of protein as does the experience of healthy vegans of all ages.
The main protein foods in a vegan diet are the pulses (peas, beans and lentils), nuts, seeds and grains, all of which are relatively energy dense. As the average protein level in pulses is 27% of calories; in nuts and seeds 13%; and in grains 12%, it is easy to see that plant foods can supply the recommended amount of protein as long as the energy requirements are met.
Protein is synthesized by the human body out of individual amino acids. The body breaks down food into individual amino acids and then reassembles the proteins it requires.
All amino acids must be present in the body to make proteins. Those that can be synthesized from other amino acids are called "unessential" amino acids. You can live on a diet deficient of these if you eat enough extra of the other amino acids to synthesize these. Those that cannot be synthesized from other amino acids are called "essential" amino acids and must be present in the diet.
Protein that contains all essential amino acids is called "complete" protein. Protein that contains some, but not all essential amino acids is called "incomplete" protein. It used to be believed that all amino acids must be eaten at the same time to form complete proteins. We now know that incomplete proteins can be stored in the body for many days to be combined with other incomplete proteins. As long as all essential amino acids are in the diet, it does not matter if the proteins are complete or incomplete.
The amount of protein recorded on food labels only lists the complete proteins. A product may contain much higher amounts of incomplete protein that is not listed. Combining such products may increase the total amount of protein beyond the levels expected.
And, from another source (see below), here is a partial list of foods from which vegans can easily obtain significant protein:
Tempeh (1 cup) 41g (protein)
Seitan (3 ounces) 31g
Soybeans, cooked (1 cup) 29g
Lentils, cooked (1 cup) 18g
Black beans, cooked (1 cup) 15g
Kidney beans, cooked (1 cup) 13g
Veggie burger (1 patty) 13g
Chickpeas, cooked (1 cup) 12g
Veggie baked beans (1 cup) 12g
Pinto beans, cooked (1 cup) 12g
Black-eyed peas, cooked (1 cup) 11g
Tofu, firm (4 ounces) 11g
Lima beans, cooked (1 cup) 10g
Quinoa, cooked (1 cup) 9g
Tofu, regular (4 ounces) 9g
Bagel (1 med. - 3 oz) 9g
Peas, cooked (1 cup) 9g
Textured Vegetable Protein (TVP), cooked (1/2 cup) 8g
Peanut butter (2 Tbsp) 8g
Veggie dog (1 link) 8g
Spaghetti, cooked (1 cup) 8g
Almonds (1/4 cup) 8g
Soy milk, commercial, plain (1 cup) 7g
Soy yogurt, plain (6 ounces) 6g
Bulgur, cooked (1 cup) 6g
Sunflower seeds (1/4 cup) 6g
Whole wheat bread (2 slices) 5g
Cashews (1/4 cup) 5g
Almond butter (2 Tbsp) 5g
Brown rice, cooked (1 cup) 5g
Spinach, cooked (1 cup) 5g
Broccoli, cooked (1 cup) 4g
Potato 1 med. (6 oz) 4g