I just checked my 501B (which is probably similar). I don't have a clip, so I don't know what yours looks like. You could probably buy one somewhere. The spring on mine is soldered onto the switch. Use a pair of tweezers or very small long-nose pliers and unscrew the aluminum retaining ring inside the tailcap. Resolder the spring, and your light should work again. I've replaced a few switches sourced from here and there and made them fit various flashlights. That, too is a possibility.
Conical springs are hard to come by. I save springs from dead flashlights & SureFire lamps. You could buy a cheap switch online and transfer the spring to your light.