There are several places on Disney property to buy the pins. I would recommend buying a starter kit, which comes with the lanyard and an assortment of pins. This seemed to work out the best for my boys.
Then after that she can ask CM's if they would like to trade a pin with her. Honestly, in our experience it seems like usually the CM's will ask my boys while we are in line or what not. They rarely have to ask. I think there info in the starter packet says you are supposed to limit to trading 3 pins a day; but I'm not sure how they would actually enforce that.