In order to find a PUK code online, a user can sign in to the provider's website, go to the Wireless section, select the phone for the code, click "Unblock SIM Card" and receive the code on a new page that opens. The user can then use this code to unblock the SIM card on their phone.
A PUK code is a phone unlock code. If someone repeatedly tries to type in the incorrect PIN on their phone, the phone locks up, and the PUK code is necessary to gain access. This is a security measure that many major phone providers have to help protect the devices and privacy of phone owners.