The quest item Sida's Bag dropped from the very first black ooze I killed for the Digging Through the Ooze quest. I remember this quest taking forever. Guess I just got lucky. :
Slavering Ooze - NPC - World of Warcraft
All the different bags and where to get them in WoW Classic
Muculent Ooze - NPC - Classic World of Warcraft
Digging Through the Ooze - Quest - Classic World of Warcraft