To complete your collections, you need to trade with other users. Sometimes you might be missing a critical Flamedeer and another user might be looking for something you have.
You can post a request in the dedicated thread if you have something you want to trade and search for other members who have the Reindeer you miss!
You can then search for their nickname on the games trade page and choose from your collection and theirs the trade you want to offer.
Once an exchange has been accepted, it's forever! Make sure you know what you're agreeing to when someone offers to trade you: if you trade a card you've only had once, we can't fix it or reverse it for you.