What power conditioners can convert 230v to 120v?

Check out the RGPC Powerhouse. This handles 208-240v and converts it to 120v. This is a MONSTER and it is a total solution for power conditioning but it will cost you...
It's msrp is $9000US. It can be had for less.
It is all you wil need for your entire system though

To tell the truth, not much audio equipment (or other stuff) is "designed for the US". It's a global economy, and compatability with the various voltages around the world is usually a design requirement, and an easy one to meet. Transformer primary windings are usually split into two coils, connected in series (for 240V) or parallel (for 120V). A switch or a jumper often makes the change. but some units need to have wires reconnected.

Another approach common in low current equipment is a HF switching power supply which takes a wide range of voltage (like 50 to 260) and 50 or 60 Hz.

Investigate the capabilities of your equipment before investing in a step-down device.

Eldatford is right ( as usual ). Get in touch with the manufacturers or dealers back home. It will save you a lot of money and if need be hunt up a good technician "down under" to do the necessary conversion. Generally it is no big deal.

Vamrhein,
The biggest problem you will run into is the power frequency.
Australia power is 50 HZ.... Check the US equipment, if it is not 50/60 HZ rated you could find the equipment is not a bargain after all.

60 HZ only power transformers will hum louder and fail in time due to excessive heat if connected to 50 HZ power mains. Equipment with motors in them may run slower.

Try to avoid "converters". They are usually cheaply built autoformer-based. rather than a true transformer. If you do require a step-down transformer, check out used items on Ebay. I just picked up a regulated one for $29 that would have been over $1200 new and their lifespan is often decades. You would need to know the maximum amp draw of the components and multiply by 2 (roughly).