Hi Shondell!
Thank you for visiting the Moms Panel!
You will be able to buy milk (in small cartons) at your Quick Service restaurant, the Intermission Food Court.
If you wish to buy larger quantities, there are a few grocery store options:
Take a taxi to the Goodings Supermarket, located fairly close to Downtown Disney. It is best to take a taxi as it is at least a 30 minutes walk from 'Once Upon A Toy' at Downtown Disney, and you have to walk past all seven Downtown Disney hotels and then cross a large intersection, and go past McDonalds.
For a wider selection of groceries, and possibly a more affordable price, you may like to take a taxi to Publix on Vineland Avenue (again, you would need a taxi, due to the distance from Downtown Disney).
If you only need a few essentials, you could try the HESS Station, which is just across the street and very close to Downtown Disney.
You may find that your Food Court is perfectly adequate for your needs.
If you have any other questions, please come back and ask me - I would be happy to help you plan your magical vacation!