Acid gland drops from Soldier Ants and Larva.Soldier.
Soldier Ants can be found inside an Ant Hill.
Larva can be found near Spade Gulch. The entrance to their cave is next to the Spade and is blocked by Acorns. After destroying the acorns you will encounter a Field station, just keep going straight until you hit a path blocked by quartzite. After breaking it with a hammer you will find a swarm of larva. Be careful as its likely they will attack as a group.
There is one more entrance on the other side but is quite difficult to find