Hi Nick,
as far as I know they can not be purchased any more.
There was a guy in Gippsland custom making a similar unit, but he has stopped also.
I found some second hand units off old A class's

after I had set up I beam on my new boom

, so local boy's are using those

.
I am using a short section of I beam, with roller car. A lot heavier, but it certainly works under load, mind you I am using 4 to 1 purchase on it

.
Windslyce make a low profile recirculating ball car and track for outhaul similar to mainsheet track, that may also be a option.