Best bet is a wreckers both in terms of cost and because you can check to see the one you're buying is ok.
Holden spare parts take an arm and a leg AND your first born for a new one. Secondhand they don't come up on eBay very often (plus you can't actually test them before you buy).
For Melbourne try these places (not an exhaustive list obviously):
Allcomm Wrecking (Moorabbin) ph. 9553 2511
Comcar (Reservoir) ph. 9462 1988
Commodore Spares (Clayton North) ph. 9543 2444
East-Comm Wreckers (Ferntree Gully) ph. 9758 2955
Holdent (Clayton) ph. 9544 9155
Holwreck (Coburg North) ph. 9350 6821
Just Commodore Wreckers (Epping) ph. 9408 4766
I'm familiar with Comcar i was there recently - they should have some.
Not sure about price, but i'd imagine around $60-$90 each (my wild guess, as new is in the hundreds)