Search internet for mobile phone unlocking websites. You will come across a thousand websites who can do this. Choose which ever you like. Do look for price comparisons and the unlocking instructions.The best way to unlock your mobile without vain follow the instructions here http://www.mobileunlockguide.com/ and unlock your mobile.
Recheck the Model number- Network to which your phone is locked to and the IMEI number before placing an order. If you made any mistakes in these details these people will never give you a refund!!
One more thing, before placing an order ask them about the conditions for getting a full refund in the case of a code failure! Think and make a wise decision.
I have personally bought the code here http://www.wickedunlock.com/apple-iphone-4/rs11wp9/ unlocked my mobile. . It is because they have good reviews on most of the mobile phone forums. I got my phone unlocked successfully from that website. I think their codes don't fail.