For equipment as heavy duty as 500 Nm, you will not find "hobby" style stores that carry these.
You will have to look at industrial supply companies that specialize in motion control.
Also, for that level of torque, you may need to look at AC induction motors, as permanent magnet based motors may not be available that can achieve that.
How fast do you need the shaft to spin at? Achieving 500 Nm at 1 rpm is going to be a lot simpler than achieving 500 Nm at 100 rpm!
It may be that the power you need requires you to go to electric vehicle type motors. These typically require drive voltages higher than 24V -- the Nissan Leaf runs at 400V, and has 280 Nm of torque. But, again, what speed you need is an important factor.
Places that sell this kind of equipment can be found on Google -- just look for "motion control."
Expect to spend many thousand dollars for the motor, and as much more for the control/inverter circuitry.