What a great head start she has, 50 pins, wow.
We trade all over the park. Just be on the lookout for a CM with a lanyard. We've traded with other guests too, but mostly CMs.
At the Premier Shop in Tomorrowland, a bunch of professional traders hang out, mostly on the weekends. You'll find a mixed bag of pin trading there. Some just love to trade and will trade anything at all with the kids. Others will only trade the more valuable pins, with you giving up your valuable one and them giving you a less expensive one. Then there are those that won't trade at all with other guests, they only trade with CMs. (On one trip, my son a had pin that another guest really wanted, but wouldn't trade with us. As soon as he saw us talking to a CM, he came running up ready to trade with the CM because my son was going to give up the coveted pin. I wouldn't let me son trade it, I just didn't want that other guy to get it.)
You'll always find a CM with a lanyard (or 2 or 3) at the pin carts scattered throughout the park. Also, more and more other CMs are starting to wear lanyards now too. The black lanyards are for everyone, and teal are for kids only. Maroon lanyards are manager lanyards and usually have better pins. Just remember to always ask to see pins, don't just start reaching for someone's chest.
You can find lanyards to purchase anywhere they sell pins. They are about $10-12. We started out with some I found at the dollar store. Then I got some from ebay for $4. My son now needs 3 lanyards, he's collected so many. Its very addicting. Have fun.