From the German perspective this is a pretty interesting question, as Einsteckläufe are not registered here. If you cannot find a dealer who sells them regularly in the US, you might want to contact someone who imports firearms parts as anyone who can import a barrel should be able to bring in an Einstecklauf.
I would, however, pay attention to what kind of gun the barrel is supposed to go into as not all break actions can stomach an Einstecklauf equally well.
As for good manufacturers, I can recommend Keppeler who make fantastically accurate ones as well as Keller und Simmann which are easier to adjust, but didn't prove as accurate as the former.
