This guide will let you know where you can get Growing Power in Warframe. Go through the guide and follow the step-by-step instructions to get it.
Growing power can drop from Knave specters which you can find on earth in the silver grove by spawning them using the nightfall apothic. You will need to have the silver grove quest to find the silver grove and craft the nightfall apothic.
In this mission, you first need to capture the target and then look around the entire map for the silver grove. When you get near the silver grove, the lotus should start to comment on it or say something about it.
The mod does have the lowest drop chances out of possible mods; the specter can drop 50. It will likely take you a few runs before the mod drops for you unless you get lucky on the first few runs.