Inside the parks, just look for any Cast Member. Some stores have books the kids can look through. I am pretty sure guest services has a "pin stroller" or at least they used to. It is not in the open and you had to ask for it. You can trade up to 2 pins per cast member and of course helping the kids to ask politely and with thanks goes a long way in the pin trading world. Cast members that have green lanyards trade ONLY with kids, whereas all Cast Members with pins are open to trade with.
Outside of the parks Disney Springs is a great place and sometimes there are special areas set up for pin trading. Good Luck!