Dark green, leafy vegetables are the most important source of plant matter. One of the reasons for this is that these deeply pigmented vegetables are a great source of vitamin A. Any well stocked grocery store should have a wide variety of greens such as kale, romaine lettuce, Swiss chard, watercress, endive, bok choy, escarole, spinach and duckweed. Other items that should be used are thawed, mixed frozen vegetables such as peas, corn, broccoli, cauliflower, etc. Do not feed iceberg lettuce, it has almost no nutritional value and many animals prefer it to other more nutritious foods. Don't forget seasonal foods such as dandelions, either.
Many people have more dandelions than they care to deal with in their yards.
As long as they have not been sprayed with an insecticide or weed killer, they are an excellent source of nutrition. Iguanas will eat both the green leaves and the flowers. The other plant matter that the iguana will love is fruit. Colorful fruit, especially red and yellow fruits, seem to delight iguanas. Again variety is very important.