I guess I have had much better luck than you have buying them on eBay including finding some unused ones.
I have 6 in use currently and several spares.
Don't recall ever having a button failure either so lucky again I guess. Although I have not bought one in some time I still have a saved search on eBay that sends me an email when one is put up for sale. Usually get 1 or 2 notifications a week.

An MX-600 is the same as a 500 with additional RF capabilities. They can clone from each other.

IRClone was a 3rd party device and software that allows you to program the remotes on a PC and clone the setup to the remote and do much more. The files you see here:
[Link: files.remotecentral.com]
are IRClone files.

This functionality is the main reason I continue to use these 20 year old remotes.