There are few AU companies with Apple or Mac anything, plus
a few resellers in regions that may or may not have any deal-
ings with older hardware and obsolete models.
There is a listing online for applebits:
http://www.applebits.net/catalog/index.php
And a forum of various older Mac users:
http://68kmla.org/forums/viewforum.php?f=13
So you may have to consider contacting OWC in the US,
or some other parts suppliers of compatible optical drives
that have correct specs for a slot-load drive, be it Combo
or just CD. Depends on the kinds of media you use and
whatever the machine could support.
An external USB optical drive probably won't work due
to the slow USB1 speed. If the computer had FW ports
then it would be a candidate for a FW400 combo drive.
More costly & harder to find; and tray-load iMac G3 may
not be supported.
A newer machine may be the answer, if anyone there
restores them at an affordable price. The shipping + tax
is part of the big problem across the Pond to Oz. I see
a few in places who refurbish Macs in the US.
Good luck & happy computing! 🙂