I have an abundance of acid. That's due to the following factors:
1. I bought it at every opportunity on my traders;
2. I also bought wheels at every opportunity, in addition to the wheels I looted;
3. I picked the chemistry station as a tier completion reward (I didn't craft it before because I couldn't find a beaker -- and I still haven't found one);
4. I finally got the acid book (the last one I needed, iirc).
There is just one medical container that can be harvested for acid, which is the first aid one. It's a white box with windowed doors, and can be lootable or empty. I believe you can get acid from both versions. It is not wrenchable, so just break it with an axe. The acid book also let you harvest acid from cars, which is a good source. The barrels that give acid are the green ones, iirc. The ones on that POI everyone seems to have died at lately, the house with a cemetery underneath right before the loot room.
But after you have your vehicles and chem station there isn't a lot more you can do with acid. The only recurring resources that use it as Learning Elixir, Moonshine, Recog and Sham, none of which seems worth spending acid on.