An online shop called Track of the Wolf has lots of stuff like that but they are in the states. Try Forbes gun shop or whatever it is called. And I think there is another in Adelaide if you search blackpowder. In your wandering if you find a savage smokless muzzle loader let me know
The sale of kits in Australia is virtually banned for private shooters. A dealer or gunsmith can bring them in but they must be submitted for inspection and testing by customs before they go to the buyer.
Given all that hassle, just buy a nicely completed one off the shelf. If you want cheap, there are various CVA and Dikar reproductions on Usedguns.com or just buy a quality gun like a Pedersoli, I have a matching pair of 50 cal Kentuckys in flintlock and percussion.
If you want to hunt with Black Powder you may also consider a Black Powder Cartridge rifle in various configurations, my 50/70 Sharps will handle the biggest varmints!
Hey BAW, welcome to AV, good to see you over here, for those that dont know, BAW is what might be loosly termed an expert in all things to do with the evil black smelly stuff, single shots, casting, etc, etc, etc. He doesnt mind sharing his wealth of knowledge as well. You could almost call him a good bloke.