I went through this last spring, and I was surprised to have trouble finding used XT hubs. Perhaps I just looked during a dry spell, but there was nothing on eBay that was even remotely attractive. I ended up buying a new XT rear hub and was similarly surprised by how expensive it was, and that even for new XT it was fairly hard to find the non-disc version. I wanted non-disc because, as far as I understand, that adds a little more dish, which I wanted to avoid because I don't foresee ever wanting to use the wheels with disc brakes.

Again, maybe it was just a dry spell (I could certainly imagine that the highest demand for such hubs is in the spring), so I'll be curious to see what you find.

Good luck!