As long as you play the whole game in singleplayer, you do not need to search for them. You will see more then 5 through the story progress.
To kill them, you should use some explosive weapon like the grenade launcher on him. If they have enough damage taken (Something about 3-4 hits), they are starting to crouch. Now its your time to go behind them, and make a takedown with
. Be ready for rapid pressing a button then. After that, they are dead, and you will earn 3,000 credits for each.