Used is likely to be just as bad as the one you're trying to replace... They're not exactly a commodity item, I really don't think you'll have much luck if any at all.
What exactly is wrong with it? I've had the bearings go in the guide bushing of one of my machines and I could have cooked an egg on the casting it got so hot. Anyway, we pulled it out and rebuilt it - if it can be made it can be rebuilt. That's probably your only option aside from biting the bullet and ordering a new one.