You could try Craig Deacon from KegSystems. He mainly deals in new kegs but I believe that he does have access to old ones, well I know he does as he loans them to brewers for the beer awards as part of his sponsorship. Mainly 20's and 10's but he may have access to 50's. We bought some new 50's from him a few years back.
Probably, yes. But most won't carry them.
One of the retailers here imported a bulk-lot of 50L kegs from Europe a few months ago, but sorry I can't remember who they were.
I'm thinking ubrew.com.au for some reason?
Some people have been known to "pick these up" from behind their local boozer. Some say this is liberating a repressed keg from the cluches of mega brew.
Others say it's stealing.