Unfortunetly your best bet would be to buy one through the trading post. There's not too many second hand bike stores in Australia and Sydney isn't exactly bike Friendly. In fact in a survey done by Cycling Central, Sydney came LAST out of all the capital cities when it came to ease of commute and cycling infrastructure.
But I'm digressing. The Australian trading Post (www.tradingpost.com.au) and maybe even cash converters (a chain pawn broking store) will probably have something of interest.