In food products, trans fats perform very similarly to saturated fats. For example, a pie crust would not become flaky with a liquid oil as the main ingredient. A solid fat is more stable and keeps layers separated during cooking. Hydrogenated shortening works as well as lard for this purpose. Hydrogenated fats or solid animal fats are often select for deep fat frying, because they stand up to the heat better.
Hydrogenated oils are also in convenience foods.
Margarine is a primary source and, as mentioned above, so are vegetable shortenings. Most commercially prepared baked goods such as cookies, cakes, pies, donuts, and crackers contain hydrogenated oils. And, they are prevalent in commercial fried foods, such as French Fries, fried fish and fried chicken. It's probably not possible to completely avoid hydrogenated oils. But, if you use hard stick margarine, shortenings, many convenience food products, fast foods and mass produced bakery products, you are probably eating a lot of trans fatty acids.