First, there are always a few modules at any shop, that do not have the source code. Source code recovery is a project in itself. There's no magic by which you get the source back. It turns out that the company I worked for, did outsource this work to one such firm with source-code recovery expertise. We all had our load-modules(and dbrms) shipped to them in the xmi format over e-mail. After months, they sent us the source code, and I was happy.
This is not just a mainframes-centric phenomenon. I once had my manager, pull his hair out when they had lost the source-code of a few DLLs, of an application on MS Windows platform, and the client had asked for a small change.
Thank you very much.
Software Engineer, Lives at Borivali, Mumbai