TK discusses a portable power supply module known as a Battery Shield for Raspberry Pi and Arduino.

Another answer is to find a higher voltage supply and mate it with a cheap power supply module. We’ve also seen non-PC power supplies put in a PC case.

The power supply is a switching design with some filtering on the motherboard. One of the boards of the power supply module rectifies the incoming line voltage to a high DC voltage (about 400 volts).