The stuff is purchased from Honest Jimmy's House of Stuff with 50 pieces of eight from the Trouble Brewing minigame on Mos Le'Harmless. Access to the minigame and shop requires completion of the Cabin Fever quest. "The stuff" apparently contains some questionable and/or toxic content, as it has been banned by the Cooks' Guild of Varrock. However, it does not actually cause any negative side effects when used in brewing.
Portal (Trouble Brewing) - OSRS Wiki
Map of OSRS wiki views per capita : r/2007scape
Justine's stuff for the Last Shopper Standing - OSRS Wiki
User:Joeytje50 - OSRS Wiki
Gabooty's Tai Bwo Wannai Cooperative. - OSRS Wiki
Shantay Pass Shop - OSRS Wiki
OSRS Wiki on X: OSRS fun fact #10: Goraks have the weirdest, most broken drop table in all of RuneScapeunless you play with your pet kitten?! Here's why: / X
Amlodd's Magical Supplies - OSRS Wiki
Pieces of eight - OSRS Wiki
Slayer Rewards - OSRS Wiki
OSRS Wiki now shows restock times on all shop pages! : r/2007scape
Dawn Greenman Facebook, Instagram & Twitter on PeekYou
The Deeper Lode - OSRS Wiki
Wesley - OSRS Wiki
Love the wiki but sometimes, the gear recommendations are like: : r/2007scape