The Wolfsbane appears, along with the Nightblade, in Malc's first random character encounter. When you first sight Malc this will trigger a cutscene were Malc has been waiting for his Uptown Rider friend, DeSean but hasn't met up with him yet so Malc asks Johnny to help find him. Johnny agrees to help. When you find a bike follow Malc until you find DeSean down in The Meat Quarter. He tells Malc and Johnny that some bikers were cheating in a race and DeSean got shot. DeSean then leads Malc and Johnny to the cheating bikers in Westmister were starts a brief stand off. After you have eliminated all the enemies a brief cutscene starts. After you succesfully complete this mission there are many bikes left behind by the deceased bikers, which are both Nightblades and Wolfsbanes.
During one of Angus' Bike Thefts, Angus Martin will occasionally ask Johnny to steal a Wolfsbane. (Note that the mission briefing will refer to it as a "Wolfbane.")