CAA makes them in plastic/polymer and aluminum. Here's the 1st link that popped up: CopsPlus.com: Command Arms AKT Aluminum Tube for AK
I have the Vltor that I got from Brownells... lots of other people carry them too, but many are out of stock. Vltor Weapons Systems -Modstock Accessories and Components (4th product down).
Many people report that the CAA has a gap(s) where it enters the receiver; all I can say is that the Vltor comes with shims that eliminate any gaps. The Vltor also has QD sling mounts on both sides of the tube. It's a well built piece.