Where can I get a reasonably priced iphone? I'd prefer brand new?

I'm currently on a contract with Verizon and can't upgrade til February. But was thinking maybe some way to buy a phone compatible on Verizon elsewhere at a cheaper cost than what Verizon gives, then just putting it on my contract. If that is even possible. Would probably want a 5s or 6(all depending if there are no issues with them)

Thrift shop. (I'm not a gangster.)