Speaking as only someone who's looked into it, and not done it .. the three big names that always come up are 1) The Dive Shop (
www.diveshopscuba.com) 2) Adventures In SCUBA (
www.adventuresinsuca.com) and 3) Aquasport (
www.aquasportscuba.com)
I think all 3 have top rated reputations. For what it's worth though, the last time I looked into this, Aquasport had the best prices on courses and equipment hands-down.
Also Aquasport, being in Marda Loop, may be easier to get to from the NW than the other two, which are both on Macleod Trail.