You could use a battery charger. You would have to connect the power & remote wires to the positive wire of the charger. It should work as i hooked it up that way just to see if an amp i had actually worked and it did. I'm sure there are actual convertors but i dont know of any. But you could also go to a car audio shop that has car audio equipment on display (Circuit City, Best Buy, local car audio shop, etc.).
Go to any electronics store and purchase a 12v AC transformer that matches your amp's current requirements. Then buy a rectifier and a large capacitor. Connect the transformer to the rectifier and capacitor. You can search the google for making your own simple powersupply. The problem with battery charger is that you'll get AC noise(humming) cause it doesn't have a large capacitor to smooth out the current/volts.