i found them on ebay, but they are really a light sensor. if a shadow moves into the sensor range it activates the device. Here is the link.
Talking Motion Detector Security/Play! - eBay (item 180281082882 end time Aug-30-08 12:22:01 PDT)
I have some of mine hooked up to ac/dc wall adapter, which sometimes will activate the device as soon as power is turned on.