Cellunlocker.net is a genuine service. I have bought a few codes from them. If cellunlocker is not able to get your code for you, I would try
masterunlockcodes.com - I have bought a ATT 920 code from them last year (although at that time, ATT codes were cheap and easy to get - this is not the case now). I have heard good reviews of
chronicunlocks.com, but have not tried them in personally. If none of these services are working for you, I suggest you check if your device is having a bad IMEI# due to unpaid bill or reported lost/stolen.
As xandros has said above, ATT devices are a pain to unlock now and expensive as well (the going rate for Lumia unlocks seems to be in the $30-$50 range) - I remember unlocking the 920 for $10 early last year!