I don't know if you can. Here are a few ideas.
1. Call the online Apple Store number. They probably don't have any more, and probably can't tell you about anyone who has any left. They're probably the best bet for learning about new/unused 4G iPhones.
2. Look for refurbs at the online Apple Store. I just looked, and there are none at the moment. Check every day, and you might get lucky. If you're really interested in it, be prepared to grab it when you see it. If you don't, someone else will.
3. Call a local Apple Store. They're probably out, but try anyway. You could also call a local AT&T store, but my experience with them and the iPhone shortly after it was released was that they were clueless about the iPhone. I doubt that's changed.
Good luck!