Thanks for posting your question. It is entirely possible that the source no longer exists, but if you can still download the update from somewhere (maybe try copying that URL into a browser on your own machine, etc.), you can provide it in a local source folder and still publish/deploy if you need to, as outlined in this doc.
I would test the download elsewhere to determine whether the issue is with your SCCM server or the download site, and go from there. If it is your console, you can supply any affected patches locally and still publish what you need to while you address the SCCM server's issues. Let me know if that helps.