Where can i get a unlocked refurbished iphone 5s cheaper than 500 dollars?

I have wanted an iPhone 5s ever since it came out, and my parents can't afford to pay $700 dollars for one. My mom told me she would try and get me a refurbished unlocked one (we're on straight talk, we can't afford a contract service) for my 8th grade graduation, which is in one year. Does anyone know where I can get an unlocked, refurbished iPhone 5s cheaper than $500?

Att will unlock them after 6-8 months of active service you can you the att go phone service on it at $60 mounts gives you unlimited talk text and 2 gigs of data i'm using it with my 3gs

Cell phone doctor

Ebay