You are definitely on the right track! Either one of these small trees grouped together would provide a tall, natural fence-like display. Because both are native perennials to your area, they are likely to be disease resistant and hardy. Both are popular landscape plants and should be easily located in local nurseries. Check our list of suppliers to find one. Let's look at each of these trees in detail.
Magnolia virginiana (sweetbay) produces fragrant, 4-6 in. white flowers which are open during daylight in the long, hot summer. Its attractive, glossy leaves are evergreen in the south. It likes a lot of water and thrives in part-shade. The growth rate is slow to moderate eventually topping out between 12-20 ft. Find more information about this tree at our wildflower site, Flora of North America, and USDA,
Ilex opaca (American holly) grows slowly to a range between 12-36 ft. tall. It thrives in part shade with medium water use. The female tree produces small white blossoms from March through June. Both male and female specimens are necessary for blossoms. With multiple plants in a hedge having both types should not be a problem. The lovely red berries amid the pointy green leaves are characteristic of the popular holly decorations used for the December holidays. Note that the berries may be toxic to humans if ingested, though the birds and butterflies find them quite to their liking. Read more about this tree at our wildflower site and USDA.
Two other possibilities for consideration are Viburnum prunifolium (blackhaw) and Rhododendron maximum (great laurel). Both grow well in part shade and are attractive to birds. Rhododendron maximum (great laurel) is evergreen and will grow to 15 ft. Viburnum prunifolium (blackhaw) will also reach 15 ft. but is considered deciduous. Any one of these four will provide you with a long lasting barrier fence.