Go ask around for ANVAS accredited nurseries within or around the Darwin/NT region and ask for certified disease free Sharwil avocado planting material - ANVAS nurseries are typically wholesalers so they might not have as much staff available to wait on phones or keep an eye on emails as they are usually busy out in the field but their prices are not as steep as retail or online nurseries.
Easier to source certified Sharwil on disease resistant rootstock from within NT than shipping from outside the state - isn't NT, TAZ and WA now behind a green curtain?