Just do missions, there’s no faster way
There actually is a few ways to stock up pretty quick. First method is to loot the museum at Philippi Battlefield Cemetery. You can get 3-5 stims per server hop. There are a few laying around and there is a locked med box near the cash register that can spawn with them, the Pharma finder perk boosts this. Second method. North/NW of Big B's gas station is an unmarked building that spawns a bunch of ticks. Server hop to farm for tick bloods. Use chemist perk to double up the stims you craft. Butchers bounty may still affect tick blood drops, not sure. Also, server hop and shop player venders. Some high level players only use super stims and sell all the regular ones they get.
Actually there is a “fast” way, but it requires Big Pharma perk (you find it in luck), the higher the level the better - now just go in the Huge airport (I forgot the name sorry) where you usually fight Scorched while waiting for a cargo bot and loot all the medkits you find. If you lucky you can earn 6-10 stimpaks this way; just rinse and repeat in all medical/scientific outposts you find in apalachia and you should be able to get a huge load in few time! Regarding ammo, Super Duper perk gives you a small chance to double the ammo you produce in the tinker workshop; there is also a public workshop (called Converted Ammo factory) on top Nord Ovest of the map that, upon being claimed, allows you to produce ammo you need for free