> going to a seedy part of town, picking up a dropped cigarette butt, and leaving it at the crime scene I've sometimes conjectured collecting hair from barbers

> going to a seedy part of town, picking up a dropped cigarette butt, and leaving it at the crime scene I've sometimes conjectured collecting hair from barbershops and making a dusting bag that steadily fluffs out hairs to dirty up a crime scene. Maybe get some saliva from grocery store sample spoons.