Where can I find the blueprint ?
Everywhere, basically.
You can’t. I told the devs to remove the blueprint from your copy of the game.
Hiya Lovecraft and welcome to the forums and Grim Dawn.
Basically blueprints can drop from anything that breaks and any mob, or you can try the trading threads as listed below. Good luck 
Also the plant vendor in the Ancient Grove roguelike sells blueprints for non-Mythical legendaries, so if you visit him (it) often enough, you might come across that particular BP.
It also sells mythical BPs, just not guaranteed.
Oh, I didn’t know that.