So something you can do now to get the tree ready for a new round of grafting is to start spraying it with copper sulphate about 6times yearly.
This will kill any fungal infections hiding in the bark.
Thats what killed your grafts before. Yes its very important to remove the tape as soon as new growth is detected.
Now regarding the actual scion material.
There are two ways to get the material you need. First is someone living nearby provides some cuttings.
The second way is to buy a new tree that will grow in your area. That is, the rootstock is suitable for your area and the varieties grafted onto the dwarfing stem stock are known for sure to tolerate your cool climate and be self fertile. Once the tree is settled and doing well with new growth, begin taking one scion at a time only, and grafting it onto your big tree. Only take one scion because removing more material can shock the new tree and kill it. I know thats a more longer term solution but might end up being your best bet in the long run.
Back to the first way.
There are very strict biosecurity regulations throughout Australia.
For example, nothing can go to Tasmania without considerable prior efforts and paperwork. Plant material must go via a specific plant transport agency so it can be quarantined and sterilized and the history of the material must be completely documented.
Eastern states can move material around but I think avocado, being a commercial primary industry, is a controlled material so only registered nurseries can move it around.
Bananas used to fall into this category also until mid last year when the scientists finally lost control of the infectious diseases running rife in people's backyards that plagued the industry from time to time. Now its up to the farmers to be aware of the pitfalls of moving material around and introducing new material to an area.
Nothing can go to Western Australia from eastern Australia.
Unless you lived within 100ks of me in the subtropics, I would be reluctant to provide material if I had any.
I don't have avos because they are too hard to grow in my area.
So the way I see it, buying your own tree to harvest from is probably your best long term bet. That young tree will also be disease free as well which is a huge bonus.