Would a sledgehammer to a tire work? I saw Fedor doin' it and been wantin to try it ever since.
Those are great for rotational strength and conditioning but I'm not sure how directly relevent they would be to slams. Either way though they are good exercises
I would also suggest a heavy band workout. Tie off the heavy band and start pulling over your shoulder in a very fast explosive movement. I first saw it on a Jeff Monson clip and I've been hooked on power bands ever since.