Ordering via amazon maybe a option,
Rearz out of Canada sells through amazon and they have a good selection of premium adult diapers and abdl clothing.
Incontrol is the medical side of Rearz and they two are on amazon.
I do not now for sure if amazon australia would carry the above.
To deal with delivery issues you could use Amazon lockers.
Another option is Walmart which also carries the above.
You could have the order shipped to the store near you for pickup.
Now that being said.
Diapers are often considered to be medical device and need to be certified by a national health agency for them to be sold in that country.
So if the Vendor does not get the product tested in Australia it may not be available there.
If there is someone in Australia that is manufacture quality adult diaper they most likely are selling them through Walmart or Amazon.
Best of luck.
PS
It most likely be best to get you wife on board because it you start are going to wear diapers it may get hard to explain why you smell like wee when you come home. Just taking the Diaper off does not take the smell with it. If she understand others Kinks you might be surprised what she is open to. A woman's need to mother a child is very strong and that could work for you. In your other kinks is she the dominant, See how far she is will to go with regards to controlling you.