I ordered this variable voltage power supply module:
Ultra-Small Lm2596 Power Supply Module Dc / Dc Buck 3A Adjustable Buck Module Regulator Ultra Lm2596S 24V Switch 12V 5V 3V #*******
This tiny little power supply is smaller and more powerful than anything I could build myself. It's also very simple to use, with clear, simple markings on the board for its connections. And I couldn't beat the price, either! Everything was awesome.