Usually its by a Pin code. You take it to stores who does unlocking and they will do it on the spot. Or you can submit your IMEI number to them and they will get back to you with the code. There are other methods too. Ofcourse, all the above cost $$. It usually range from $30-60 approx ( not more than $80 for sure).
Browse through 
Howard Market place and find the store that offers you a good deal in unlocking it
Upon quick browsing at HOFO, here's 
one
( apparently the blackberry cost more..normal cellphone go for less around $40-50)
Good luck!