Hello and welcome!
I live in Seattle so I can't go to the parks very often, but when we go, we are always on the lookout for CMs to trade with pretty much everywhere. Custodial staff often have lanyards, and they usually love to stop and chat. CMs working in the shops, especially World of Disney, also often have them (but we're always polite and wait until they are finished with other customers first, or if they're a cashier we get in line and wait our turn.) Also several of the stores have a pin board or lanyard behind the counter, including World of Disney, Westward Ho, and... I want to say Elias & Co as well? I might be wrong on that last one. My best friend's better at remembering this than I am. As long as you're nice, the CMs don't mind you asking, and will direct you to which counter has a pin board if they have one.
Also some of the employees in Downtown Disney have lanyards, particularly at the Disney Vacation Club booth.
Also if you stay in any of the Disney hotels, the Guest Services desks have a pin board, and the gift shop too (at least they do at the Grand Californian, which is easy to stop by via Downtown Disney).
Happy trading! 🙂