The most accessible is likely various pages on the
Sarna wiki, although this is by no means comprehensive. The original data most likely comes from the various Handbooks for the different nations of the Battletech universe, or other random sources that mention certain companies.
Known news organizations include, but are not limited to:
ComStar News Bureau (major source in Succession Wars era)
Interstellar News Network (mostly takes over from CNB in the Jihad era and following years)
Federated News Network (Federated Suns)
Irian News Interstellar (Free Worlds League), subsidiary of Irian Media Interstellar
Donegal Broadcasting Company (Lyran Commonwealth)
Tharkad Broadcasting Corporation (Lyran commonwealth)
Solaris Broadcasting Corporation (almost entirely Solaris-focused, as might be expected)
Capellan Wire Service (Capellan Confederation)
I wasn't able to find one based in the Draconis Combine, although they almost certainly have at least one.