New cell phone, no committments: Vonage.
Used cell phone, no committments: ebay
Also Tracphone is a good option, you prepay with your credit card and use it up - I think you buy phone cards to insert? or enter on the phone? Something like that.
At mobile phone recycling companies
Searching mobile phone recycling companies and compare the top one, to get the best price.
on eBay
Amazon.com
No, not unless you need a specific feature that is only in the latest phone.