Plenty of them on ebay, and check your local craigslist. I got my ATP nine years ago for $300 on craigslist, has worked fine ever since. The price of used ATPss seems to have gone down with the advent of the Equinox, Vanquish, and the Noktas.
How about a garrett Apex instead? I have one that only has a few hours on it. It's basically brand new and still has lots of warranty. Let me know if your interested.