There is one available specifically for the Sigma, it's a Streamlight M4 if my memory is correct. I have one so if you can't find it under that name let me know and I'll get off my lazy butt and look. It fits the Sigma specific rail.
If you're not going to be subjecting it to a lot of travel, movement,etc as I'd expect from a home defense gun one of the "one size fits all" lights would probably work just fine and probably save you some money. I think the retail on the Streamlight is $150 or so. I found mine on either gun broker or ebay for about $89 if I remember right. You might want to look at both of those sites.