It has been out of stock for longer than that I think.
Some very good substitutes are:
Nordic Components... They now have an expanding mag tube and are pretty much the standard when it comes to mag tubes
TacCom3G. Their mag tube extension is a heavier gauge (thicker!) aluminum and you may not need a barrel clamp for it. DID I MENTION THAT IT IS REALLY THICK?!?! None of the aftermarket barrel clams will fit it except theirs.
Lancer has a very good carbon fiber tube as well