I don't know of anywhere in Australia that you can get a feather bonnet, and a quick google search isn't turning up anything. However, if you're willing to order online, you can try here. J. Higgins, Ltd., a US Company, also has them. Also, anyone who supplies L&M Highland Outfitters products (they make some nice sporrans, and you've probably seen them), can probably order you one from them.

Of course, feather bonnets weigh a ton, cost a mint, and are the devil to maintain. They're also only really appropriate if you're going to wear full "Number 1" dress with doublet, plaid, horsehair sporran, spats, etc. That's one of the reasons pipe bands don't really wear this style of uniform anymore (c. 1970, it was /all/ they wore). You're talking a good $3000+ US to outfit one person in full dress. That's a lot of piping gigs for a solo piper.

Anyway, good luck with whatever you choose!