Good quality puppy food such as Nutro Puppy is good for pregnant and nursing mothers. You can get a 50lb bag of 12% protein sweet feed from the feed store for around $7. Make sure it doesn't have any whole corn in it. They love the molasses and the grain is good for them. Black oil sunflower seeds are good too, as are all sorts of cereals(no sugar). Some fruits and veggies are good, but too much can cause diarrhea. You can use frozen peas and corn, they love them, as well as some beans(lima, pinto). You can even put some of the dried split peas in their mix.

Hope that helps