Here are just a few online sources for powdered milk, corn for grinding into cornmeal, and spelt (a grain similar to wheat).
Milk:
Provident Pantry Instant Nonfat Dry Milk
Emergency Essentials
Country Cream Instant Nonfat Dry Milk
Grandmas Country Foods
Maple Island Instant Nonfat Dry Milk
Alpine Food Storage
LDS Church Cannery Nonfat Dry Milk
Provident Living
Nestle NIDO Instant Dry Whole Milk
My Brands
Morning Moo Low fat Dry Milk Alternative
Augason Farms
Corn:
Whole Corn for Grinding (not Sweet Corn)
USA Emergency Supply
Walton Feed
Spelt:
Wheat Grass Kits
Emergency Essentials
Purity Foods