Not sure if it's common knowledge but in the Magic Kingdom both city hall and the candy store across from thh emporium have books of pins behind the desk. All you have to do is wait in line as ask POLITELY. The pin store in Frontier Land has a "trading post" its a wooden post on wheels that they bring out from the back sometime that is filled wtih pins.
In addition on International drive is one of disneys character warehouses. You can pick up pins there including starter sets very cheaply. These pins are "legal" traders. The usually have some starter sets and older holiday sets that were left over from the parks.