The newest phones are pretty tricky to unlock. Stands to reason really that the manufacturers are making them more and more secure, because as you probably know they substantially subsidise the cost of the handset.
If you had an old phone that you wanted to unlock, there are tons of online resources to help you do that such as
this one but if it's a case of buying a new phone, I would agree with gj76 that your best bet is to buy one here.