Honestly, I've just periodically bought them in stores whenever they pop up. It's not rare. Like 3 here and 4 there. They appear on the tables in stores. Just honestly spend the time to buy them that way, dude. There is no bulk option for stimulants that I have seen in over 60 hours of playing. The game kinda puts you in a speedrun mindset and it doesn't need to be that way.