-12v supply from +12v
There are a number of ways to get a negative, below ground supply.


Easiest: If the source is AC, uses a full-wave bridge connected opposite to that of the positive supply.
If DC only is available, use an inverter that may use


a charge pump for low currents or
an inductor or transformer for higher currents.



The Texas Instruments TPS54620 is a step-down (buck) converter, which can be configured to provide a voltage below ground, e.g. this circuit from Digikey.

An Analog Devices LT8331 could supply 80 mA at - 12 VDC.



An inverting charge-pump such as the Microchip TC1044S can 20 mA, if that meet your needs.

It is even possible to use the 555 timer/oscillator to get -12 VDC, though regulation would not be good.