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Replacement key caps
from Ducky, as far as I know, are only available as full sets. You can
find them here, including the recently-released sets for back-lit keyboards in three colors (black, grey, and violet). The back-lit sets, unfortunately, do not come with Windows logo keys, but the laser etched sets do (if you're willing to go without back-lighting on those two keys). It may be worthwhile to email the staff at mechanicalkeyboards.com and ask if it is possible to get the key caps depicted on Ducky's web site, though shipping costs to Australia will likely far exceed the price of just a couple key caps.
There are many other resources for key caps, and they will fit your Ducky even though they are manufactured by other companies. It is unlikely that you will find a set of back-lit Windows logo keys, but you may at least find something that you like better than the Ducky logo. While shopping, keep in mind that the Windows logo, menu, and modifier keys on your Ducky are the 1x1.25 size. A 1x1 size key cap would fit, but would look pretty silly.
This page at keyboardlover.com has a good list of vendors, many of whom offer individual custom and novelty key caps, and there's lots of other useful info on that site, as well. Keep an eye on the Group Buys threads here at GH and over at DeskThority.net, as you are more likely to be able to find exactly what you want that way. Be aware, however, that group buys tend to take several months from inception to delivery, and can be a little confusing.
The Vendors forum is also a good place to look. Feng and imsto, in particular, typically have a nice selection of key caps from China and offer worldwide shipping. Feng's selection can be browsed on
his eBay page, but you can get a lower price by sending him a PM here on GeekHack. Best wait until early next year, though -- he's a student and very busy with final exams at the moment. I've not yet bought anything from him, but word 'round the campfire is that he's an excellent business person and you won't be disappointed. I'm also interested in the
red silica gel O-rings offered by imsto, and his thick PBT key cap sets.