It's kind of up to cast members whether or not they want to wear one of the pin trading lanyards. They are supposed to have them everywhere...attractions, stores, etc. So it really just depends how many CMs feel like putting one on!
Like Dana said, you can trade with any cast member who is wearing a lanyard. I guarantee once you're looking for it, you'll find them everywhere!
But the best places will be like the Green Army Man store in Tomorrowland, because almost every single CM who works there wears a fully-stocked lanyard!
My lead at Honey I Shrunk The Audience used to wear his every day, and he refreshed the pins frequently. He LOVED trading pins with guests as much as they loved trading with him! Sadly, he no longer works there...but I'm sure there are lots of other CMs just like him!
Have fun!