"Citric acid occurs naturally in such fruits as limes, pineapples and gooseberries. The dry, powdered citric acid used as an industrial food additive since the early 19th century... has a less appetizing source; it is manufactured using a mould that feeds on corn syrup glucose."
source: The Globe and Mail.com
This is almost without fail GMO corn syrup glucose, for those who care about this. There are enough corn protein molecules remaining to make some sensitive corn allergic folks react to citric acid.
What foods you may find it in:
This is being used more and more by today's chefs to 'heighten flavors and bring balance to a dish,' so you'll likely come across it in restaurants. Also often used as an ingredient in many processed foods, like sauces, salad dressings, popsicles, frozen vegetables, chips, medications (like children's liquid tylenol). Even some of our unprocessed foods are now utilizing citric acid because it can help inhibit microbial growth, so it's used on things like those little white packets that come in the styrofoam trays holding raw meat.