Corn, like peanuts, is processed into hundreds of products that are in turn incorporated as ingredients in thousands of food products. Yep, if you are allergic to either corn or peanuts, they are very hard to get away from.
To get away from them, always buy unprocessed foods and prepare them yourself. Raw hamburger should be OK, but fast food hamburgers could have anything in them. If you want a piece of cake, you have to make it yourself with all pure ingredients. Never use a cake mix, as it too can have anything in it.
Pure lard and pure butter should be all right, but oleo margarine and cooking oil again could have anything in it.