You can buy an Iphone through ebay or even through the apple store outright.
If you want to make calls though you'll need a cell phone service to make the calls.
I'm not sure if it's possible but straight talk has a Bring your own phone program so you might be able to hook up your Iphone through straighttalk no contract wireless service but I'm not 100% sure on that.
You'd have to ask straight talk if you can run an Iphone on their service.