Disclaimer, I have never unlocked a blackberry, but looked into it for a friend.
Easiest way would be to download something like this, but don't know the cost so can't be sure if it is cheap:
https://appworld.blackberry.com/webs.../29634?lang=en
Crackberry.com often has links to websites that offer such services. No idea how cost compares though. I remember them giving away unlock codes a couple years ago though.