Not sure what the problem is for you, but it works just fine for me. Were you logged in properly (Avsim requires
registration, which is no cost, so it shouldn't be a problem)?
Other than that, there is no one place that is the definitive archive for Orbiter addons. Several of us have developed
personal databases with the locations of all the addons that we know of, but, to the best of my knowledge, none of these
databases are publicly accessible (but you can ask). If an addon is not on either Orbithangar/Spacesimmods or Avsim ...
google is your friend.
I'm not sure what the status of Orbithangar is at the moment, other than you can search for an addon and download what you
need. However his rework on the site is probably not complete yet so a bit of patience is in order as well as a little bit of
restraint in downloading files.