Not all buy from the cattle owners, and you can't be sure if they haven't mixed anything artificial or added more water. I tried this but I couldn't manage it as I don't own a fridge, so now I buy milk from the super market available as tetra-paks, which can last for over a month. There are companies like Amul, Mother Dairy, Nandini, etc., that procure milk from farmers, process it and sell variety of dairy products, like milk with different fat levels, ghee, paneer, butter, curd and so on. These work on cooperative structure and also provide additional support services like artificial insemination, cattle-feed, etc. The cattle-owners are doing a business so one can never be sure if the cows/buffalos are treated well. I think, they won't be killing it but they make sure that it produces more than enough milk, and I am not sure what they do after it dries up. Maybe they sell it to a butcher or let loose like a street animal as maintaining cows past that stage would just add to their expense. So, a cow is a sacred animal but it is a part of a business too.