Because a 9mm AR is blow-back, there is no gas tube or gas port.  In RK, they used a standard 9mm barrel with the FSB removed, and used a standard carbine-length float tube, which Chow screwed on by hand to hold the barrel in place.  You could do this yourself very easily; there are no special parts involved.  Of course, no one in their right mind would disassemble an upper like that for use as a "sniper rifle."  You'd keep the upper assembled to ensure accuracy.  The 8" of length you save by disassembling the upper doesn't make up for a missed shot.

I'm not sure the thumbhole stocks (of the type used in Leon, anyway) are available anymore.  They were originally made when it was thought that using a thumbhole would get around the pistol grip issue like it does under the '89 Import Ban.  BATF ruled that the '94 AW ban was different, and that under that law, a thumbhole was still a pistol grip.  The market for the AR thumbholes evaporated after that.  JP Enterprises sells wooden target thumbholes and "strap connect" "thumbholes", but nothing like the "early '90s import thumbhole" as pictured in Leon.

-Troy

Ok, so you can still get the thumbhole stock...  

-Troy