How can I find the older facebook messenger app

I've got a number of older Macs, from the 80s to 2015. All working, most of them networked.

I've got one of the old iMacs, the flatscreen with the hemispherical body, running OS 10.4.11, and a 2009 Mac Mini Server, running 10.11.6.

I'd like to set up at least messaging, maybe even A/V communication between them, just to see if I can.

Of course, iChat's no longer usable from the iMac as far as I know. Incidentally, if anyone knows some way to reconfigure this or that setting to get it to work with Apple Messenger or even FaceTime or Skype, that'd be great, though that's a bit of a long shot. Or maybe some sort of network-only messaging: as I recall, iChat included a Bonjour option.

Anyway, barring some surprises about iChat, FaceBook messenger seems my best option, but unfortunately although the older versions would work on 10.11 and possibly 10.4, apparently we can only get it through the App store now, and you can only download the current version of an app that way. Kind of sucks; when downloading from a company website you can usually get hold of an older version, but I don't see an option to do that on Facebook.

So, does anyone know where I might find an older version? Or for that matter, how to find older versions of any software when the App Store is seemingly the only option?

Thanks