I'm sure you can pick up a used TiBook out there. But unless you really have to spend well below $1000, why would you want to? Hardware-wise, the best of them are easily rivaled by today's iBooks, except for the screen. No warranty, old software, etc.
If it is the 15" screen you're after, I think you will see great deals on the current 15" PBs not far around the bend. And who knows what MWSF will bring. Just get the best laptop you can afford.