Here's a link to someone that posts "frequently" and "recently" mentioning he has "barrels" for sale. He very well may NOT have yours but he could be a very good lead! He's also got an image explaining the measuring of the inner barrel. His explanation is the correct method I believe.
http://forum.e46fanatics.com/showthread.php?t=738848
He's posted similarly on several forums!
I did see on a different forum he mentioned he can get ANY size he doesn't have ----- NEW !!!!
http://www.lateral-g.net/forums/show....php4?p=267619