I need to factory unlock an iPhone 4S I bought online, originally locked to Sprint?

Does anyone know from experience of a place that factory unlocks for a reasonable price? Please no comments on how I shouldn't have bought a phone online, I just need info on getting it unlocked so it can be used overseas. I tried calling Sprint and they said the original owner had to call in, but I can't get ahold of him anymore, however they stated it is unlockable.

Hi, please do not search a third-party service for this. There are only scammers. We have been to this business since 2010 and all the unlocks for Sprint I have met were all SCAMs. Please beware. A good warning from an old unlocker.