By "unlock" do you mean that you've actually locked the SIM (e.g. by entering a SIM PIN incorrectly too often), or unlock the phone so that it can use a different provider's SIM (often referred to as "SIM unlocking" but is actually unlocking the phone rather than unlocking the SIM card)?
If it's the first, you'll need to speak to the provider to get your unlock code (PUK). The SIM is always the provider's responsibility.
If it's the second, which I think is what you mean, the usual way would be to get an unlock code from the original provider while you are still their customer (they've no obligation if you are not their customer). There are people who offer unlocking services, the trick is finding someone reliable. Some phones have other tricks you can use (which vary between phones, and do not always involve root), but I don't know about the J3.