Among the animal food sources that contain tryptophan, dairy products such as (cottage) cheese, quark and whey proteins are particularly recommended. Meat and fish also contain larger amounts of the amino acid, but this varies considerably and can be lower when heated than when raw.
That's why dogs should also eat plant sources of tryptophan: This particularly includes legumes, nuts and oatmeal. Carbohydrate-containing foods, such as potatoes and grains, contain smaller amounts of the amino acid, but they stimulate the release of insulin, which means that tryptophan and other important substances can get into the blood more easily.