You can use any power supply that delivers a higher current then the charger delivers to the battery. The voltage is what you need to watch out for. And make sure the output of the power supply says 'DC' and not 'AC'.
Another idea is to get a small 12 Volt battery, like you use for a lawn mower. There are cheap and last for some time. But a little on the heavy side...
But even better would be - toss the stock charger and get a decent one. They are well worth their money and your batteries will be treated better.