This guide will tell you where to farm for Tranquil Cleave in Warframe. You have to follow the simple steps in this guide to farm it.
Tranquil cleave can drop as a rare drop from killing arid or frontier butchers, which are the grineer butchers that can spawn on either mars or earth in just about any node/mission. They spawn fairly common among the enemies as well.
The mod has a low drop rate, so bringing a nekros/any loot frame is recommended when farming for the mod.