You can get your sim at the airport or at any of the store in the city.
We bought the optus prepaid card, which cost aud $2 a day. Minimum topup of $10 each time. Which means, I am allowed to use in blocks of 5 days. As I will be staying for 6 days plus, and we reach Perth quite late in the evening, there is no reason to get a Sim on the first day. We bought our sim on the second day at hay street optus.
There is 500mb which is quite sufficient for us as we have wifi in the hotel. We used google map constantly for our road trip, the connection was great even when we travel to quite far from town.
information on $1 sim
This has only 40mb, too little unless you have not much intention of using the internet at all.