Hi!
Couple years ago I was looking for the same thing as I wanted to work on the oil rig really bad. After long research and recommendations from few commercial divers I was getting ready to join this school
Commercial Diving Training - Divers Academy International's world-renowned and sought after reputation for producing professionally trained, highly skilled divers continues to be an integral part of our success. www.diversacademy.com
I don't know if You are planning to go to US but it's supposted to be #1 place of this kind in the States. Martin Stepanek graduated from there. It is located in New Jersey not far from New York and during the training You get to work in "real life conditions" meaning cold and dirty water. The only problem for me was the cost, I only came up with enough money for tuition (including some financing) which back then was $12k but You also need not to work for five months and have enough money for rent, food and... NY nightlife
Right now it's probabbly even more so it's expensive but most of diver's academies keep similar prices, it will be hard to find anything cheaper(at least in the states, there are also schools in Texas) The Good thing is that they assist in finding a job and most of their graduates gets contracts right after certification.
good luck!
