Has the phone been off the network that it was last used on for more than 90 days? If so, you can call the network, giving them your name, IMEI code of the phone in question, and a contact number and they will give you the subsidy code free of charge.
You don't even need to be a customer.
It seems that networks are required to do this and are able to give each person up to five codes per year. However, if you need more than that, all you have to do is use a different name and contact number, and there is then essentially no limit.
To find the imei number of your phone just enter *#06# and the numbers (15 digits) will appear on your screen. This is the code that you need to give to the network.
The phone has to have been off the networks system for a certain amount of time - generally 90 days but that could vary.
Normally when you search for unlock/subsidy codes you get lots of offers from websites that will unlock your phone for anything from $10 upwards. They always ask for your imei number. This is because they are doing the exact same thing I just told you about. They call the network and request the code which they are given for free, then they give the code to the paying customer and he thinks the company is amazing!
Save money and time by going direct, they are very helpful, and it takes very little time to do.