A good quality puppy food will give your shepherd the extra calories she needs to produce good quality milk for her puppies. Do not do a sudden switch of food or she could get upset stomach and develop diarrhea. There are several good food brands on the market. Some are expensive and some are reasonably priced. Regardless of whichever you decide to buy, stay away from the cheapest no name brands since they are just junk with little nutrition and a waste of money. Many good foods that I would feed my dogs are Nutro and Natural Choice. IAMS and Eukanuba are good too. These foods cost a bit more. The cheapest food I personally would buy is Purina Dog chow or Puppy chow. Regardless of whatever food you end up buying, mix it half and half with the original food that you have been feeding to avoid upsetting your dogs stomach.
Puppy food is what she needs because it is high in fats and other nutrients she needs to support the pups purina puppy chow would be your best low cost choice since she will be eating more, and soon the pups will be eating it to.