I don't know of any shops in Australia, but heres an online service that offers japanese and other PSN prepaid cards. "Play-Asia.com" Hope that helps.
idk about a specific shop 2 buy them from. u can probably buy them online. i know a website that sell psn cards but am not sure f they sell japanese psn cards. heres the link n case u wanna try it: http://www.maximuscards.com/ . btw my bro always buy his psn cards from this website & he says that it's a safe website.