They come available from time to time, but as you have probably discerned, most Prius owners are pretty happy with the vehicle. They also are all relatively new so there's been little reason, up to now, to sell or trade them in. That said, with the 2004 coming out this fall I expect we'll see lots of the '01-'03 Priuses start to show up on the used market.
Finally, I'd just find your favorite used car dealer/used Toyota dealer and let them know you're looking and to call if one comes in. But realize this, the blue book on a Used Prius is only about $1000 less than a brand new one--they really haven't deprciated much at this point. Give serious consideration to buying a new one off the shelf.