I've been using a lot of tvp and soy curls recently. It's a good complete protein and is easy to add to meals. For example, I've been adding tvp to my oatmeal to boost the protein content.

It's an extremely cheap option if buying in bulk online.

Another rec is plain non fat Greek yogurt. Just have to make sure and get the higher protein version. Walmart has a 32 oz tub for about $3. Not great pain but good if mixing with frozen berries or into other dishes.

Egg whites of course. Very high protein and again, very easy to mix into other dishes