Buttermilk has a slightly sour taste. I don't think it is a simple mix of milk and cream?
No, it's produced by a culture, in the same way a yoghurt is. The suggestion to thin down yoghurt makes sense, although the flavour will be slightly different. Although 'buttermilk' was traditionally the liquid left over after making butter, all commercially sold buttermilk is now manufactured through fermentation and has nothing to do with the production of butter at all.
In Australia, it's usually sold in 600ml cartons, and as the others have noted, in amongst the specialty & flavoured milks at the supermarket. Even our local corner shop sells it, so it can't be that uncommon - but don't look for it amongst the full size cartons that hold 1 litre or more. I use it mostly in pancakes, it gives them a lovely flavour, and I have a devil's food cake recipe somewhere that uses it too. But I've never drunk it...
In regards to cream, it something I think has become more confusing recently, as local companies have adopted terms used in other countries but they've used them for different products. Until fairly recently, the only cream you'd get in an Australian supermarket was your bog standard 'thickened cream', with 35% fat and probably some gelatine. That was pretty much it, and as a kid I was often mystified by UK recipes that specified 'single cream' and 'double cream', neither of which translated directly to our 'thickened cream'. Then came 'lite cream' - which many people found out the hard way that you can't whip - and you also have your gourmet producers (such as King Island) with their delicious and decadent "pure" creams with a fat content approaching 55% (comparable to the British clotted cream). I know many people in Australia think that when a UK recipe calls for 'double cream', it's referring to the solid, high-fat "pure cream" such as the King Island product - when in actual fact the normal supermarket 'thickened cream' is more suitable. While King Island have changed the name of their product from 'double cream' to 'pure cream' to avoid this confusion, other manufacturers still continue to use the terms 'double cream' for a thick, spoonable 50%+ fat product that is nothing like the UK 'double cream'. I've also seen local companies producing products called 'single cream', 'pouring cream', and 'double cream' but they don't seem to be the same as the products in the UK either, although I could be wrong.
But basically - if you just want to whip some cream, then your standard supermarket 'thickened cream'- with 35% fat and probably some gelatine- will be best.