Hi Ashley,
rear sprockets smaller than the normal size may be offered by different companies. As far as I know regulary they are made of Aluminium, i.e, they are lighter then the original but probably will have a reduced life.
Christoph Theuring too asked for this in the discussion forum in july 2005, maybe you like to contact him! In response to his question concerning this item several adresses (all in Germany) were reported:
- Motorradteile Sieg, Kirchstr. 44, 64560 Riedstadt, +49 6158 73535, http://www.ls-motorradteile.de/
- Ketten Theiss, Alsdorfer Strasse 7, 50933 Köln, Telefax 0221-95440211
- Zweirad-Technik F.März. Vither Str. 43. 54595 Prüm-Niederprüm, www.yamaha-f-maerz.de

Anyway, I guess in Australia there should be any garages or companies as well who offer this service ...
Good luck,
Wolfgang