Started my fitness journey, and I have questions about protein intake

I am 25M, 5'9, and weigh 70kg. I have a belly that I am trying to reduce by walking 8k steps a day and climbing 7 floors. I follow an indian vegetarian diet, and I know that I am protein-deficient. I have also stopped consuming processed sugar and fried foods, and I can see the difference: my belly has started to shrink.

I follow a simple home workout routine with two 5 kg dumbbells, 15 push-ups, abs exercise, and squats.

I have the following questions about protein intake:

1. What protein supplement should I get? I am inclined towards Foodpharmer's protein and Whole Truth Foods.

2. Apart from protein supplements, what can I eat from my Indian vegetarian diet that's protein-rich?

3. Any cheap source of protein is appreciated.

4. I have skinnier arms, and I want to build some muscles. When I started working out I could see the difference, but it all went away quickly.