For the cheapest possible way to get a 1300 number, you can contact your current phone company and ask for one. There should be no set up cost needed since you already have existing lines with them. The ongoing rental fee for 1300 numbers are usually around $10-$20 per month.
On the other hand, if your business has money to spend and want a custom “phone word” like 1300 CALL US, then you can visit sites such as smartnumbers. Smartnumbers is an Australian government site that auctions the best 1300 numbers with a reserve of $500. For companies with a bigger budget, they can also purchase a 1300 number through sites like Telstra and Optus.
I use a Queensland company for my 1300 service. At $5.45 per month you can't go wrong, plus the payment is automatically charge to my credit card... Virtually set and forget.
Give them a try FYI Telco
There are a number of smaller providers who can offer better pricing, better customer service and a lot more features than the major providers like Telstra or Optus.
A perfect example of this is Telecube and their 1300 numbers services and features.
Definitely worth a look.