Seeds are sold at variety of retail locations. Home improvement stores and hardware stores feature garden departments that sell inexpensive seed packets. They feature rotating racks with a variety of seed packets: vegetables, flowers and herbs. Chain stores either have a dedicated garden department or small section of a home improvement department that features seeds and gardening tools. Additionally, grocery stores often place a rotating seed display rack near the produce department---ask your produce clerk for gardening seeds.
Nurseries sell packets of seeds and seeds in bulk. Rotating racks feature seed packets while bulk seeds are packaged in small plastic jars. Ask a clerk at a nursery to see the seeds and you'll enjoy a professional introduction to top-quality seeds. If a nursery employee doesn't have what you need, he'll be able to tell you where to look. Check the list at North America Fruit Explorers to find a nursery in your area: http://www.nafex.org/supply_source.htm.
Thumb through print and online catalogs to find nearly any seed that you desire. Seed companies peddle their goods online and many are solely dedicated to virtual sales. Look for organic and heirloom seeds in the Green People directory at http://www.greenpeople.org/seeds.htm. The university of Minnesota Libraries offer an online, public-access database of nursery, plant and seed suppliers. Search the database by scientific or common names of plants at http://plantinfo.umn.edu/. Additionally, consult the directory at http://www.growit.com/ to browse through various retail and wholesale listings. Browse and order online from the above resources or contact them to receive print catalogs.