The center of a whole foods, plant-based meal is the whole grain. In the Chicago area the widest selection of whole grains is in the bulk foods section at Whole Foods Market.
You can also order whole grains and many other products over the internet from Country Life Natural Foods in Michigan.
Vegetables and fruits are found in every grocery store. You can also shop at farmers markets or participate in a community supported agriculture program such as Sandhill Organics in Grayslake, Illinois. Better yet, grow your own!
Moderate amounts of salt, oil, and sweeteners can be a tasty part of your diet. You may need to shop at a natural foods store to purchase sea salt, cold pressed oils and sweeteners such as brown rice syrup or barley malt. A good internet source of specialty Japanese products such as traditional soy sauce and miso is Natural Import Company.
Wherever you shop, stick to your shopping list. Grocery stores are designed to make you buy on impulse, and impulse purchases are often junk food purchases. See Supermarket science: Stores use many strategies to sell you their products.