Generating -10V or -12V @ 500 mA from +12 / +24V
There are plenty of devices that can do what you want. Try searching for "inverting buck converter". Here's an image I found: -



You could even buy an isolating DC to DC converter and tie the positive output to 0 volts - this then gives you a negative 12 volt output. Also, flyback converters are useful for converting positive input to negative output.

The devil is in the detail (as with all switching regulators) so do a bit of research to check that any schematic offering can regulate from the range of input supply voltages you have specified.