They are not made by Koosh. They're pretty much those off-brand darts that are online, everywhere.

That's correct, they are just called "Koosh" because of the design of the head.  There are a few different types on Amazon now, quite reasonably priced.  And according to sellers on eBay, there are Gen-4 versions available.  I'm not sure I believe that based on the pics, but hey, I've been wrong before.

Happy hunting !

Thanks for your replies. I have seen gen 4 on eBay though. http://www.ebay.com.au/itm/Nerf-N-strike-Elite-Rampage-Retaliator-Blasters-200-Koosh-Darts-Improved-Gen-3/171303996691

Good luck mate !  $25 AU seems a bit pricey, but hey, I guess that's the price of living at the bottom of the world ?