Here is a very sturdy, very reasonably priced tactical light holder from Command Arms Accessories that will hold a Surefire tactical flashlight:
http://www.commandarms.com/product.asp?pID=40&cID=19
You can add a Surefire G2 Nitrolon flashlight to it and have a great setup that won't break the bank, and will withstand a lot of abuse -- the CAA tactical flashlight holder comes with a lifetime guarantee, and we all know how Surefires hold up under hard use:
http://www.surefire.com/G2-Nitrolon
You can also get one for a Streamlight Scorpion flashlight for the same price, although the Scorpion flashlight cost a little more.
You can get this holder and Surefire flachlight for under $65 total - less than that if you shop around...
I use this setup up on five of my rifles as backup lights (four Surefires, one Scorpion) - I use Streamlight TLR-1s, and on some, TLR-2s, as the main/primary weaponlight. That way, if you lose one to impact or whatever, or your battery croaks, you have a backup available on the gun.
The best part is, with the press of a button, the light detaches from your rifle, and you have a handheld flashlight in your hand in two seconds.... Also comes in handy if you just want to flash-peek around cover, and don't want to hang the whole rifle out around the corner in the process....
And with two flashlights, when you light up a bad guy, no way they can look past the lights.