It's a specific style of bike. Lots of couriers ride single speed steel framed bikes because they're reliable and strong. I don't think you will find one for under $200 because a lot of the time they're customised with higher quality parts. You could get a decent single speed for under $400 though. If you aim to get a decent frame you could upgrade other components if you end up riding more than you originally planned.

Most "fixies" have flip flop hubs. It's just a blanket term for that style of bike these days.

I know its more than you want to spend but if you're in Melbourne, Fuji Declaration's are good for the price this place @ Fitzroy are asking. Upgrade the pedals and you'll be sorted.