Plant sources tend to have a higher ratio of calories to protein, so it's tough if you're limiting calories. Chickpeas, eggplant, and tahini all have protein; Trader Joe's Eggplant Hummus has all three, but somehow manages to claim a lower calorie-to-protein ratio than any of its ingredients (not sure how that can work). Mushrooms are good too. Also seitan if you don't mind something a little more "processed".