Hi there and welcome to the Trading Card Database.
The best way to get people to match up with you is to load up what you want under the "Collection" button. If you prefer to just browse around, you can just click on anyone's name listed here and you can reach out to them privately with what they have. (To get to that list, I typed in "New York Mets" under the search bar at the top, then clicked on "Overview" and then "Collection Summary". Scrolling down from that link it says, "Members with cards from this team on their For sale/trade list".) Play around with the site features and if you have questions keep coming back to the Forums!
I'm mostly organizing over adding right now.