Instructions
Move around whichever level you are playing while sticking close to the walls and other safe areas where you can hide. Watch out for guards and security cameras, and avoid them if possible.
Find a guard who is alone and not surrounded by cameras. Sneak up behind him, grab him and cover his mouth. Drag him to a dark corner and dispose of him.
Search through the items the guard drops to see if one of them is a stun grenade. Collect the items even if they don't include one.
Look under the boxes and scour the rooms scattered throughout the level. Stun grenades are often hidden in out-of-the-way places or underneath boxes.
Continue searching for stun grenades throughout the level until you find one. There are stun grenades in every level, so if you're persistent, you will eventually find one.