Unfortunately S&W chose to use a non-standard rail. Streamlight does sell an M4 light for the Sigma, but then you can't use it on a Glock, XD, etc. Laserlyte sells an adapter (Part # SCP-813) for under $10 that converts the Sigma rail to a more usable system.
I have the sigma with the $10 standard rail adapter, I also have a Uni-Max laser for it that has a standard rail on the bottom of it. So you could get the adapter, the uni, then a flashlight on the laser...seems like a bit much though, I dont have any experience/knowledge with light/laser combos but that might be a better idea.