Which supermarket is that, mate? I'm a fan of both, but IrnBru and a cheap whisky is a great drink in non-whisky drinking weather.
Leo's in Heidelberg. Another good place to find L+P is the Tongan stall at the Preston Market, but I haven't previously seen Irn-Bru anywhere outside Scotland.
I found Irn-Bru for the Scottish members of the 2007 British JWOC team in Dubbo. I expect you could find it in most large supermarkets if you tried hard enough.