The amplifiers are fairly simple in design. Mains power comes in, is stepped down with a transformer to a reasonable voltage (18-22VAC), goes through a diode rectifier bridge, and is stabilized with some hefty capacitors. Yay, you now have DC. This is fed to the chip, along with audio input. A few resistors and capacitors make up the rest of the circuit, and you have amplified audio out, on the order of 30-70W depending on the chip selected and impedance of your speakers.
Several kits are available that include most of the necessary components (probably of better quality and tighter tolerances than what you’d find at Radio Shack), all you have to do is get a transformer and an enclosure, and do some soldering.