Besides used ones, I think Apple and Best Buy are the only places to get them. You may find some deals on used WiFi only versions after the 3G is released. I plan on selling mine to get a 3G, I didn't except to use it as much as I do so I thought a WiFi version would be fine for me.
A pawn shop may be a place to check, but I don't think it would sit on the shelves too long. Also, I'm not sure I would have the greatest confidence in how it ended up there - legally or not. I doubt they give you any guarantee on the reliability though.
Cavaet emptor with any "cheap" iPad you purchase. It's a hot product right now, so be very wary finding one "cheap" so soon.
Lots of people can't get one even when they are willing to pay the full retail price (and more than the retail price in some cases). So definitely not, at least in the next couple od months.