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.
Boiler (Trouble Brewing) - OSRS Wiki
Thingy - OSRS Wiki
TIL Runelite has a feature to right click open OSRS Wiki Quest guides : r/2007scape
Dawn Greenman Facebook, Instagram & Twitter on PeekYou
Shantay Pass Shop - OSRS Wiki
Barkers' Haberdashery - OSRS Wiki
Old School RuneScape Wiki
Soul Wars Reward Shop - OSRS Wiki
Wesley - OSRS Wiki
We get it guys use the new wiki instead. : r/2007scape
New feature on OSRS Wiki: it can now automatically check off the music tracks you've unlocked : r/2007scape
Farmer Gricoller's Rewards - OSRS Wiki
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Justine's stuff for the Last Shopper Standing - OSRS Wiki