Surprisingly, in the past two weeks I have seen two caldinas (gen4 3sgte) go for sale for $1500 in Adelaide and VIC (on the caldina Australia Facebook page). One with a stuffed gearbox (not needed for a mr2) and other bumper bar damage (not motor). Usually you caldina sell for above $4k - they have dropped big time in what they are selling for.
To buy a Gen2 from an importer or wreckers, and maintenance needed (non mls head gasket - extra kms of worn parts), total costs the gen 4 is starting to be a better investment, depending on your timeframe, then look out for a caldina gen4.
If you are in Sydney and need it in the next month to two, think SSS Autonotive use to be a go to place. Or try MR2 wreckers Australia.
Updated - engine and gearbox for sale added today on Facebook mr2 buy and sell Australia and nz