Congrats on the Atari. You can always get a universal power supply at most electronics outlets like Radio Shack or Fry's. Just look on the bottom of the controller and see what the DC rating is and make sure you can match it. Don't try just any converter because if the voltage doesn't match you can end up frying the thing. Check this link for starters. Good luck!
Check what power it needs and then go to Value Village, the one here sometimes has loads of power converters hanging in bags on the back wall for like a buck. Or as mentioned, Radio Shack for the universal model with various plugs and a switch for different voltages