Where can I find a relay?

I need a relay that can handle a lot of current. I'm not sure how much would be running through continuously but the most it would need is about 50 amps at 24 volts in the normally closed position. Also, I need 24 volts to trip the relay to off and cut the 50-60 amps. Can anyone recommend a good relay. Its for an emergency stop switch for an autonomous vehicle. The 24 volt batteries are wired to a motor controller which has 2 motors that top at 27 amps each. However, I need a stop switch. I want a put a persistant mushroom button to activate a relay which will also be connected to the same batteries (in parallel of course) to trip the relay to turn off the power to the motor controller. Any help would be great thanks!