If I had a Dollar for every SureFire product and option that SureFire don't list on their website I would be rich.
You can order a 917FA from your friendly SureFire Dealer or direct from SureFire by calling 800 828 8809
Do you currently use the 617FA?
If you do, all you need is an A12 extender tube, P90 Lamp Assembly & SF323AC battery 9V battery stick.
And, if you are planning on getting an M962P, have you considered an LM90 Millennium Lamp Module for your shotgun forend. This would give you parts interchargeability because both use the same Bezel and Lamp Assemblies.
The Benelli S90 (& M1 ?) WeaponLight Forend Housing is the H17FA and has a list price of about $200.
The LM90 Millennium Lamp Module is about $125.
Or
The Classic L90 Lamp Module is about $90.
I'm sure a SureFire Dealer can get you a better price then buying from SureFire direct.
Al