If you want a good one, you can get the Cambridge Model Twelve. It is a complete portable system (Also known as the Mick Fleetwood Model). The unit plugs into house current, but can also be run off of 12 volts in a car. Sooo- you have to buy a rechargable 12 volt battery. You won't need a car battery, just one that can last a couple of hours running the sub. The Model 12 lists about $500 and you can get it for about 4. You can find them used on ebay occasionally for about $275.
Why would you want to do that? Why would you want a battery-powered subwoofer? You would have to keep on remembering to recharge it regularly or when the battery gets low, and I think that the subwoofer will not be as powerful as one running on house current. I think you're best to stick with an AC powered type.