Fast Food is readily available almost everywhere in the USA and is very inexpensive. Pizza is sold everywhere. They usually sell groceries, snacks, pastries, cold drinks, coffee, hot dogs, and sandwiches made to your order. They are very inexpensive. You can usually get a light breakfast or lunch for just a few dollars.
Supermarkets sell many things besides groceries.
Look for the soup and salad bar or the delicatessen and hot food sections. They sell hot soups by the cup, the pint or the quart. Spoons and napkins are free. Make your own salad from a choice of several dozen items and pay by the weight when you check out at the cashier. Food Courts have become very popular in most shopping malls.