+-10v variable power supply
Sigh... Spehro Pefhany already gave you a great clue. Shop for a power supply that puts out +/- 10 volts or more likely +/- 12 volts. It does not have to supply even 1/2 amp to work.

Buy a 10 K potentiometer (and a knob) and connect its CW and CCW inputs to your +/- 10 volts. Connect the wiper to your motor controller. Connect the common grounds for speed control and the power supply together. Done.

The speed controller will likely ignore any voltages above 10 volts or less than - 10 volts.