Your local juice distributor (Veryfine) or others will supply the machine for no cost at all. All you need to do in purchase the products from them.
As far as who do you need to ask? Your building principle. Tell him you won't need to ask him for anymore funds if you can do it.
Who fills the machine? The robotic students. Afterschool or before. Set up a schedule. Maybe those who help can get $$$ put towards there trip.