Here are some of my contributions that may or may not be viable depending on your diet and budget:
Chicken gizzards. Crazy high protein, super cheap (local store has them for $1.50 a pound), and really easy to cook. Just toss in a slow cooker with some water and a small amount of fat (I use butter) and let it cook overnight/while at work. Can make a stew or just simmer them and eat them as is with whatever spices or condiments you want.
People have mentioned yogurt, but I am partial to the yogurt drinks from companies like Chobani. They have a no sugar added, 20 gram, 150 kcal drink that's nice and convenient, though a bit pricier than just getting a big jug of greek yogurt. $2.50 a pop at my local shop.
Fairlife milk. Ultraprocessed, a little pricey, but double the protein of milk while also being lactose free and easy to digest. Not a fan of whey protein so I find this preferable for a quick protein boost. About 5 bucks for 2/3 or so of a gallon.
Beans, more as a carrier for higher protein lean meats than anything else. A go to poorfag protein meal I used to make a lot was a can of seasoned black beans, a tin of inexpensive smoked sardines (or any canned fish), and some spicy salsa. This morning I had refried beans and a few eggs in it with sour cream and tomatoes. Diced pork chops are a good add too.
Extra firm tofu. I like to make light vegetable soups in the mornings sometimes and I will dice and toss in a good deal of tofu after it's done cooking. Don't drain or press or anything, just toss it in the hot water right after I turn the heat off. You can use as much or as little as you want, but I use plenty since a pound is usually a couple of bucks at the store.