The Staples website might not list the KDX, but the back end supply system does. And you can access it from the web.
There's a site that will let you look up the stock inventory for Staples, Target, and Home Depot. You can find it here. You'll need to enter your area code along with this code:
If you do get one, let me know that it worked. My local stores don't have them in stock.
In case anyone is wondering, I don't think this is a sign that the KDX is on the way out. Amazon is still selling the KDX for $380 new in box, and they are even selling refurbished stock for $245. I think this clearance sale came round because Staples never sold very many KDX. That doesn't surprise me; it's rather expensive and not nearly as capable as, say, the Asus eeePad Transformer.
But I also don't see a replacement KDX coming anytime soon; it's not a good design and a lot of tablets can do more for less.