These are currently unavailable, we do sell replacement bulbs, however.  Part number N7/61/C.

The handbrake test switch is the rectangular black switch on the dash with the little square red lens. When your fluid bottles get low, brake pads get low, or handbrake is on this light illuminates. Of course this all depends on your wiring. When you depress it (key on) it illuminates to tell you IT is functioning not your brakes!