It is my understanding that cast members receive one or two "Lanyard Pins" when they start a shift. They are mixed in with the other "common" pins.
Since cast members aren't likely to volunteer which ones are "lanyard pins", it is important to know what they look like before begining trading. eBay is a great reference to see what the latest lanyard series pins look like.
As for strategies to find these pins, here are a few....
1. Trade with cast members when the parks first open. If you are fast, you may beat the other hundreds of other pin traders to the punch.
2. Find cast members in secluded places away from the throng (good luck!). Examples might be small souvenir shops well away from the main park entrance.
3. Later in the day, scan for supervisors- with practice you can learn to spot them walking purposefully across the park. They may spend part of the day away from guests, and so their pins are less picked over.
4. If you can figure out where cast members enter the park, sometimes you can waylay them before they reach their posts. One example is at the big wooden fence near Africa in the Animal Kingdom.
5. One spot where I frequently see lanyard pins is at the water parks. Guests typically don't think to bring their trading pins here, and locals pin traders with Florida Resident passes don't want to pay extra to go into the water parks. This can be a bonanza for lanyard pins, especially in cold weather when the water parks are nearly empty.
That is all I can think of at the moment. Good hunting!