Beahcboy is right about the power supply. You won't find an AC/DC converter with enough power to drive the amp, or it will not be cost effective. THe cheapest way to implement your idea is to use an old stereo receiver to power the sub. Heck, you can get a 100w KHL at Best Buy for $90.
I'll warn you that it may sound like crap. Subwoofers tuned for a car with a lot of cabin gain often sound bad in a larger room. It is possible that a new box would help, it that were the case.
