If you are a breeder or rescue you can usually by from wholesalers. Just have to find the one with the food you feed.

Another way to gain access to bulk buying is to directly get in touch with the pet food brands and inquire about their wholesalers and distributors. Get the numbers of your preferred dog food brand off the net - Purina, Pedigree, Royal Canine, Orijen all have contact details on their websites.