Besides making trades within the blogging community, I also trade/receive cards from a couple of trading groups, The Bench and Trading Bases. I have been a member of The Bench since 2003 and have completed close to 500 trades. It is a safe and well ran site with lots of nice features like a trade manager. I have never been deceived or not received a trade made through the site. Unlike the Beckett site, it is free to join.
Every now and then one of the moderators from the group receives a box to break from one of the card companies. He is very generous with these cards, giving them to other members who ask for them. Topps sent a box of Heritage Minors that yielded some nice Cardinals including a blue border insert, a peach insert, and this game used card: