What is a good, cheap battery?

I want to make a small quadcopter. I plan on spending maybe around $100 or something, maybe $150. I am going to get some cheap motors, an old pixhawk, and ill worry about the controls later. For now though, I dont know where I can get a battery that can connect to the old Pixhawk. If you have any cheap suggestions, let me know! They dont have to be fantastic, they just have to work.

Consider using the 18650 battery pack