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.
Shop - OSRS Wiki
Miscellanian Clothes Shop - OSRS Wiki
Settings - OSRS Wiki
OSRSBox, Blog
Looking through the Music Cape OSRS Wiki while at the club and now I'm crying : r/2007scape
Canifis - OSRS Wiki
Daryl's Ranging Surplus - OSRS Wiki
Shantay Pass Shop - OSRS Wiki
Justine's stuff for the Last Shopper Standing - OSRS Wiki
We get it guys use the new wiki instead. : r/2007scape
Dawn Greenman Facebook, Instagram & Twitter on PeekYou
Sigmund the Merchant (shop) - OSRS Wiki
Wesley - OSRS Wiki
OSRS Wiki now shows restock times on all shop pages! : r/2007scape
Map of OSRS wiki views per capita : r/2007scape