As long as we are talking about the brake master cyl that has the brake fluid reservoir attached to its body and you don’t own NA2 NSX-R, the parts # is 46100-SL0-954.
It's not sided so same parts for both the RHD/LHD and for all year models except for the NA2 NSX-R.
If the worst comes, there is also OEM rebuild kit (pistons, etc) available from Honda but if your internal cyl bore has already worn, best to replace the whole unit.
Should be still available as I just received one from Japan here in UK.
I don't know how the parts system works in Australia but please first try a different Honda dealer.
If it still says discontinued, then, since you are in Melbourne, it would be cheaper to have it delivered directly from Japan to your address using the English speaking parts distributor based in Japan.
I have never used one by myself so can't comment based on my own experience but some of the UK owners used the following services in the past and seemed they got good services;
rhdjapan.com
amayama.com
Please note that most of the companies in Japan will be closed for their summer holiday around WK2 – WK3 in August including all Honda offices.
Kaz