A voice changer device for an Xbox 360 or an Xbox One is not commercially available. However, a device that receives audio input through a microphone connected to a computer, modulates it with voice changing software and supplies the changed audio to an Xbox can be assembled easily.
To build a voice changer you need a 3.5 mm stereo male-to-male audio cable, a 2.5 mm to 3.5 mm stereo male-to-female Y adapter and a pair of headphones in addition to the computer and microphone. First, plug the microphone into the computer. Then, connect one end of the 3.5 mm male-to-male adapter to the headset port in the computer, and the other end to one female port of the 2.5 mm to 3.5 mm Y adapter. Plug the headphones into the other female end of the Y adapter and connect its male end to the Xbox controller. Run the voice-changing software, and speak into the microphone to get a changed voice feed in the headset and on the Xbox.