You can look here by tapping Shop at the top of the page and choosing Basic Phones.
Personally, I would buy a basic phone elsewhere before I'd sign a two-year contract for one. On eBay you can filter selections by clamshell, new, U.S. only, Buy It Now, etc.
Or, if you're OK with meeting strangers, check Craigslist. I bought two fully functional basic phones for $10 each.