This question is difficult to answer because of the nature of laptops. The price of a laptop is going to vary based on how powerful the machine is as well as what comes packaged with it. It's important however for you to understand what it is from the laptop that you're going to need, in order to prevent you from buying a laptop that isn't going to be right for you. As for prices, they can start anywhere from ~$250 dollars into the several thousand dollar range depending on what you're needing.
I've written a hub about how to determine what you need in a laptop, and it can be accessed here: http://hubpages.com/hub/laptop-buyers-guide
As for where to access one, I would say head to your local big box retailer, whether that be a place like Best Buy Wal-Mart or Staples, or a wholesaler like BJ's or Sam's Club.