So I am looking at options. Amazon have a "Playstation 3 System Messenger Bag" listed, but it doesn't seem to be available here. I've had a look around all the usual New-Zealand-based computer stores, and none of them have any sort of a PS3 bag listed. I can't be the first person in the history of New Zealand to want something to put their PS3 in something when it's not being used.
It's possible maybe that a regular laptop bag will work. PS3s and laptops are distinctly different shapes though. I shall have to make some inquiries tomorrow.