Buying an iphone 4s outright?

I'm saving up for an iPhone 4s, but i don't really want to go on a contract. If i bought one outright could i put it on pre-paid for any phone company? I'd rather pre-paid over a plan so I'm not locked into a contract and i can manage my usage. Which would be best; Telstra, Optus, Vodafone etc. Would i have to buy one from the Apple Store or can i buy one from anywhere that sells them?
Also, how much would i be looking at to buy one outright in Australia?

Contracts aren't a bad deal, most don't change phone companies anyways and they pick up most of the cost.