There are a few different types of enemies that drop acid upon dying. Some of them might be harder for you to deal with depending on your setup and level. In addition to killing these mobs, there is also another way of getting lots of acid. You can do combinations of mob & resource farming.
Mobs that drop acid are Snallygasters, Wendigos, Anglers & Grafton Monsters. There might be more but these are plenty to get you started. I suggest focusing on Snallygasters.
You can always find them at Toxic Larry’s Meat’n Go. There will usually be a boss and two mobs along with it. In addition to acid, they also drop other valuable loot, plus the rest which you can loot from the place itself before re-logging and killing them again.
If you don’t want to keep re-logging to farm these, you can also take the Hemlock Holes Workshop (3 acid nodes) up north on the map, and set up your camp east of it for an additional acid resource node you can farm via an extractor (set up your CAMP there).
You can then keep going back to the other place every 20 minutes or so to kill the mobs that re-spawned in the meantime.
Another good workshop south on the map is Federal Disposal Field HZ-21.
This one only has 1 acid node but it has 3 nuclear material ones!
There is also an additional two nodes slightly north east of it that you can farm via your CAMP. One of them being acid.