Welcome to Healing Well IllyKid!! While I am in the USA I don't know where you can find the vents in Australia but I know they can be found on Amazon and other Medical Supply websites. Some have even mentioned EBay for some items.
I know there are some items that I source through medical supply sites when my insurance doesn't cover the expense and save a lot of money.
I hope someone chimes in with information in your country...if not, try the internet
Hugs to your brave son!!