Magic Kingdom is the worst park to trade for pins
Of all the Walt Disney World theme parks, you would think that Magic Kingdom would be the best location to trade for pins but it actually is the worst. I have spent a lot of time scouring for trading pins with my kids and lately we don't even bother to bring our pin books with us to Magic Kingdom.
While I am certain there are other pin trading location that I have either looked over or failed to see, it seems that the only two easily approachable boards are "Along Main Street" (see what I did there?), in the gift shops.
The first board can be found when you enter the first shop to the left of the circle at the parks entry. This is the first gift shop after you pass by the guest services building. The pin board will be located at the check out counter on the far left when you go in and you may have to wait behind a throng of other guests waiting to check out.
The next board is located on the same side but further down the corridor of shops nearer the center section that sells anniversary merchandise or limited time merch. Just ask a cast member where the board is as they will sometimes move it to one of the two adjacent shops. They may also have a big board on a tripod stand that makes it much easier to look but this isn't an every day thing.