If you can't find a local supplier try eBay for an international seller. I know Australia has really weird customs laws so check first about whether you would need a licence to import them.

Try emailing webmaster@raingardens.com They sell a huge range of Brugmansia seeds, and will be able to advise you on import rules.

Searching the Australian ebay site gave six results.
http://shop.ebay.com.au/i.html?_nkw=brugmansia+seeds&_sacat=0&_dmpt=AU_Plants_Seeds_Bulbs&_odkw=brugmansia&_osacat=0&_trksid=p3286.c0.m270.l1313