It can be tough to find used implements that are still in good shape - you're right, they are often sold with a tractor. But it can be done.
Having said that, I haven't had much luck finding them myself. I have looked at the resources suggested in this thread, and they're useful, but the problem is often that when you add shipping to the cost of the implement you're talking $$$ that is awfully close to new (or close enough that I would personally prefer not to risk the money on something that may have been used hard or poorly maintained).
One thing that sometimes works is to just keep your eyes open. Around here people often just put stuff out in the front of their places with a sign on them, and I've seen a few good deals. Problem with that is that by the time I've gotten back to 'em the implement is gone /forums/images/graemlins/frown.gif. Local papers occasionally list things like that as well, but the same thing applies - if it is a really good deal there's folks like you and me waiting to snap 'em up.
Good luck in your hunt...