I buy locally sourced branded milk it costs about $4.60 for 2L. I personally think this is really important for animal welfare as well as the farmers. It also in my opinion tastes much better.
I think it is very morally questionable for supermarkets to sell milk so cheaply that dairy farmers can't afford to survive. I feel the same about meat and fruit/veg as well and wherever possible buy those from small producers at the markets.
On the other hand I know not everybody can pay that much for milk. My sisters family drink 15+ litres of milk a week and buying cheap milk isn't really optional.
What makes me annoyed is seeing people with $1 a litre milk in their trolley and then litres of expensive branded soft drink or even water!!!!
I think if it's an option people should 100% by locally sourced ethical milk but I totally get many people can't.