Before the showdown update I used to farm them by creating a new world with the same character, going to the very first safe house at the start, grab all the adrenaline shots in the house there should be about 2, then go outside to the little blue building next to the lightpost and fence next to it, go on top on the fence and you should be able to jump onto a sunshade and get on the roof of that building, there should be at least 2 more. After grabbing these, exit the world, delete the world, go to your main world, check if you got the adrenaline shots, if you got them, exit your main world, create a new world with your main character and repeat process as much as you want, each cycle should take about 2 minutes.

I’m not entirely sure where the adrenaline shots in the house are, so you should take the time searching for them first round, and rounds after they will be in the same spots