Specifically, 835 and 837, and even more specifically, lists of the various status codes and their meanings.
I've found a couple of commercial sources via google and duckduckgo. Are any sources freely available?
Thanks.
The specifications are copyrighted by DISA or WPC (in the case of the HIPAA standards -- which are essentially an augmented version of the base X12 standards) and available for purchase from them.
As Axel suggested however for the more popular transactions sets you can usually obtain implementation guides from the net, and these will generally have all what you need.
There are generally not machine readable versions of these specifications freely available, as the official versions cost a great deal.