idk if anyone will see this but hopefully it helps:
To bulk buy stimulant you must progress the events with Adrianna (a dwarf girl) until you can access the underground secret dwarf base, go left(ish)-down of the main hall and take the room you see first off the right of the underground where you will find a blacksmith area, the dwarf at the anvil will offer stimulant as well as other useful items like coal, iron bars and mithril bars :)
Here's the link for Adrianna's events: Adrianna - Renryuu Wiki (miraheze.org)