This guide will let you know where to get Shred in Warframe. You have to follow the simple instructions in this guide to do so.
Shred can drop as a B rotation reward from nightmare missions; you can only get the B rotation nightmare on certain planets.
The planets that can give you B rotation rewards are Phobos, Ceres, Jupiter, Saturn. the Void, Europa, and Deimos. But if you run a nightmare mission on a planet Phosc and don’t get the mod, you have to go to a different planet because you can only claim nightmare rewards from a planet once and then wait till it refreshes.
It may take you multiple runs until you get the mod.