Unfortunately Amithlon was stopped due to legal disputes. The only way to get it these days unless youre lucky enough to find a 2nd hand copy is via the usual "questionable" sources on the internet.
I bought my copy of Amithlon from old unopened stock that Software Hut had not too long ago, maybe about 2 years or so. They might still have some still in stock Tasmanian Guy, give them a call, or write Joe an email and ask him if he has any left to sell.
It is a damn shame that the Amithlon project ran into legal problems and disputes between the two primary developers that were working on it, which brought it to an abrupt end. It is an even bigger shame that no one has picked up the ideas contained in the Amithlon project and continued them in a new project title.
Maybe some day in the future some bright computer science student will pick up where Amithlon left off and create an even better Amiga(Like)OS that runs natively on modern hardware and retains backward compatibility with most of the Amiga apps and games, but such a project will probably never happen as it would be a huge waste of time and effort.
I hope you can find what you are looking for Tasmanian Guy, but I think you are better off with AmigaForever as it will run more of the old Amiga software than Amithlon can and runs on a wider range of hardware too.
