I suggest that you call the stores around you that you would ordinarily also shop for the rest of your groceries and ask for their prices. You don't want to go to a store where the milk costs less but then do the rest of your shopping elsewhere. You'll eat up your savings in gas and time. Therefore, also notice when you first shop where you're wanting to get milk, the prices of their other foods.
I've found, over the years, that different grocery stores have lower prices on some things and higher on others. I choose which store to use based on the average price of all the foods I buy as well as it's location and convenience.
My daughter buys organic milk as well as some other groceries at Trader Joes. She does her main shopping at a Fred Meyer that is located just a few blocks from her house.