I am not sure where you can get a single evergreen seedling this time of year. Most nurseries that have them sell them in quantity for replanting or reforestation purposes. Sometimes conservation districts and garden centers have tree seedling sales in spring. Try contacting a local garden center (sorry, I am not permitted to recommend specific firms) to see when they will get their seedlings in. Even though it is getting late to do so, you might find a small leftover potted evergreen tree that was intended as a tabletop holiday tree.