You need a high-amperage "120V AC to 12V DC convertor". However, these are rather expensive (>$250 for 50A AC/DC convertor).
Or, if you are powering a very, very small amplifier, you can modify a PC power supply to allow you to do it, but because most PC power supplies are rather low amperage, you are very limited in the amount of power you could run off one of these.