Which software/website should I use to factory unlock my AT&T iPhone 4S? - 1

I need to unlock it to be used on T-Mobile. I want a permanent unlock and I was wondering if any of you have done it and what your suggestions are? Thanks (:

Details
I am buying it from a family member. I have never had AT&T, always T-Mobile.So, AT&T won't unlock it for me. I wanna know a good website that does it for $20 or under.

Ill unlock for $20
email me

You can get it permanently unlocked for $10 here; http://gsmunlocklabs.com/unlock-iphone/

all you need is your IMEI, a new SIM, itunes and that service. One time and you're done and the phone is permanently unlocked. It's super easy.