just to clarify, you don't *need* an arcadevga - it just makes things a lot easier.
Provided you have a card that advancemame supports (check its docs), advancemame, IMO, gives a very good result for a fraction of the cost

. I've also recently got Raine's arcade monitor mode working (directly through my trident blade card) and am very happy with the display obtained with that method.
As far as hardware goes, yes, you'll probably need a video amp. Buy from ultimarc (or specs are out there to BYO). Ultimarc's amp also comes with a vga cable, meaning you'll just need a cable to go from the amp's "out" screw terminals to the connector on the monitor chassis. This will probably merely be a matter of hacking together your own cable with the appropriate connector for your monitor.
Regarding purchasing a monitor, bumper action
www.bumper.com.au (in Melbourne) were selling some new cga monitors a while back fairly cheap- check their parts pages. You'd probably be better off trying somewhere in Adelaide though - check the yellowpages under "amusements"
Also, Jomac in perth (I think) sells re-furbs- they fixed my chassis at a reasonable price anyway . . .
I'm sure you'll turn something up. Good luck!!