I made most of my own baby food with a food processor & freezing in ice cube trays. I would buy frozen or fresh produce from places like Sprouts, Whole Foods, Central Market, Costco, Sam's, Walmart, Target, Kroger, Tom Thumb, etc. Try to look for sales & buy what's in season.
I would make mostly carrots, green beans, squash, potatoes, bananas myself. Some of it like applesauce I would buy in a large container.
You can buy grains, beans, lentils, pastas in bulk. You can also get plain yogurt & cottage cheese in larger containers. My son also likes eggs. Oatmeal is a good inexpensive breakfast.
For milk, Braum's is better than most non-organic since they don't use the growth hormones. Most stores sell organic milk. Shop around to find sales. We now get fresh goats milk.
Food co-ops are good too since a group of people will split the cost & get a bigger discount for buying a bulk order.
You can also look for local growers in farmers markets, roadside stands, etc. We're growing a lot of produce in our backyard garden now. Here are some links for some information & local growers you might find of interest.
http://www.motherearthnews.com/
http://www.localharvest.org/organic-farms/
http://www.picktexas.com/
http://cbs11tv.com/wrapper_consumer/moneysavers/Food.Coop...
http://www.oklahomafood.coop/otherstates.php
http://www.coopdirectory.org/directory.htm
http://www.crosstimberscoop.org/