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.
Petrified Pete's Ore Shop - OSRS Wiki
Inaccessible areas - OSRS Wiki
Love the wiki but sometimes, the gear recommendations are like: : r/2007scape
Slayer Rewards - OSRS Wiki
Slayer Rewards - OSRS Wiki
Fairy shop keeper - OSRS Wiki
PvP Arena Rewards - OSRS Wiki
Honest Jimmy's House of Stuff. - OSRS Wiki
Music - OSRS Wiki
Justine's stuff for the Last Shopper Standing - OSRS Wiki
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Thyria's Wares - OSRS Wiki
Trouble Brewing - OSRS Wiki