Here's a place to buy the pins, and also a whole kit.
Or just the pins;
Your "tumbler" will need to have paddles inside it to insure the media actually tumbles. Just sliding around inside the pail won't do anything. Also, 5 gallon pails are tapered enough to fit inside each other when empty. This makes them not "track" straight when rolled on it's side.
Then there's the problem of sealing them water tight. And no, the media HAS to be wet, and the liquid must have some citric acid in it. That's what changes the black stains back to brass color.