I have always bought BR7ES, BR8ES and BR9ES at any Canadian Tire for over 10 years. They are readily available off the shelf in the spark plug section. Look for the NGK Sports.

On a side note, I run BR9ES Solid Cores, gapped to 0.016", stock ignition system and sh!t loads of boost.

I change them when I do an oil change. Approx ~ $50 for a jug of Castrol 10w40, 4 fresh BR9ES's and a new Mitsu filter is the service my DSM gets