Right now I have a circuit which connects a battery to a motor which propels an 80 pound robot forward. I'd like to add a simple pushbutton to the circuit that allows me to instantly make or break the circuit. It should be push-on/push-off (neither state should be normal?) and I should be able to easily hit it or step on it without worrying about breaking the button or missing the button (and so being ran over by the robot if, say, the reason I need to stop it right now is because it's about to hit me.)
I feel like for something with such simple requirements I should be able to find such a part for ~$10. Where do people normally shop for these kinds of components, anyways? Surely you don't manufacture every part of every robot you build?