Any iPhone purchased officially in the US requires a 2-year contract. Unless you hit up ebay or craigslist you won't be able to find one without a contract. And even if you do it will only work with AT&T, so you will need to jailbreak and unlock it.
That question was already answered. All iPhones sold in the USA are locked to AT&T, if you want to use another carrier you will need to jalbreak it and unlock it. See above.
Or you can go to an Apple store and pay the no commitment price, and leave the store with an unactivated iPhone. However, it will still be locked t at&t