Q: Where do I get A USB-C cable for Apple TV (4th generation) or a Micro-USB cable for Apple TV (3rd generation or earlier)

Any electronic stores, Amazon, heck you may even have one already From another phone or camera. Just google the product and you'll see tons of results. Do you have the new ATV4?  then it's the newest USBc connector. And depending on the your computer with iTunes, you may need the USBc - USB 3.0 or your computer may have a USBc connector so the USBc-USBc would suffice...

http://www.amazon.com/Type-C-Male-Cable-Nokia-Black/dp/B00SWP1EGE

http://www.amazon.com/gp/product/B00WAKL4TY?gclid=CNCBw63ok8kCFQsDaQodWU4EyA

here are the instructions in case for restore you dont already have it...

Restore your Apple TV using iTunes - Apple Support