If you're patient / lucky, you can find someone with a BNIB (brand new in box) selling it because they can't return it, or used it for a day, or something like that...
If you use Craigslist, just do the exchange at a Verizon store so they can note it on your account, verify the ESN, etc. You can also check out Swappa.
It just depends on what you want. If brand spankin' new is most important, then as huskur / bsweetness already said, Verizon or Amazon are probably your best bet. 3rd party retailers like Best Buy will always be higher off contract. But if you're ok with "gently used" (but still like new), it could save you $100 to $200, but obviously "caveat emptor" in that situation.