I used coal tar pitch (came with my kit) to make my first mirror which was 8” f9 back in 1986. The pitch you listed components for is called tropical lap. I had enough coal tar pitch to complete my mirror so didn’t need an alternative.
Whatever pitch you get, buy extra in case you need to pour a new one whether it is too hard, too soft or simply set on you too quickly or you need to pour a new one if you’ve left the project for too long. If you get coal tar pitch use a pot you don’t want as it’s never going to be used for anything else again. The whole house will smell like roadworks pouring new bitumen. I recommend you get a portable gas or electric single element hot plate and heat it outside to avoid fumes in the house.
I found that Koppers is the only Australian manufacturer of coal tar pitch but where you can get some I’m not sure. Perhaps email them.
See
https://koppers.com.au/carbon-materi...cals/#CMC_News
(Scroll down).