If you're willing to risk a used device for a (potentially) lower price, you might check out the online auctions at Goodwill. Their selection of ereaders reflects the market -- the biggest share is Kindles of various generations, and those fetch the highest prices. They always have Nooks of various models, usually have Sony and Kobo models, etc. They often have "off brand" models, but you have to be there at the right time.
They are sold without any guaranties and there are no returns, so there is a bit of risk, but I've bought several and never had a major problem. I think most auctions are for US customers.
http://www.shopgoodwill.com/listings...1&ending=Items
They do, also, have a tablet section that might be worth checking out. But if you were looking for a tablet, say 7" or smaller, I might check Amazon or Newegg.
http://www.shopgoodwill.com/listings...5&ending=Items