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. :
All the different bags and where to get them in WoW Classic
Black Ooze - NPC - World of Warcraft
Glutinous Ooze - NPC - World of Warcraft
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Cursed Ooze - Wowpedia - Your wiki guide to the World of Warcraft
Sida's Bag - Item - Classic World of Warcraft
Primal Ooze - NPC - World of Warcraft
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All the different bags and where to get them in WoW Classic
Grungy Containment Pack - Item - World of Warcraft